add support for ViewGroups with custom onLayout()

A custom GtkLayoutManager is set to these objects, which calls into the
java handlers when measure or layout is requested.

Androids onMeasure method is quite different from GTKs measure method,
because Android already defines the final size during onMeasure.
Therefore, we call onMeasure from GTKs allocate callback instead of the
measure callback.

src/api-impl/android/view/View.java

@@ -771,6 +771,14 @@ public class View extends Object {
 	private Context context;
 	private Map<Integer,Object> tags = new HashMap<>();
 
+	int measuredWidth = 0;
+	int measuredHeight = 0;
+
+	private int left;
+	private int top;
+	private int right;
+	private int bottom;
+
 	public long widget; // pointer
 
 	public static HashMap<Integer, View> view_by_id = new HashMap<Integer, View>();
@@ -829,7 +837,8 @@ public class View extends Object {
 	}
 
 	protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
-		native_set_size_request(MeasureSpec.getSize(measuredWidth), MeasureSpec.getSize(measuredHeight));
+		this.measuredWidth = measuredWidth;
+		this.measuredHeight = measuredHeight;
 	}
 
 	public Resources getResources() {
@@ -846,6 +855,9 @@ public class View extends Object {
 	protected native long native_constructor(Context context, AttributeSet attrs); // will create a custom GtkWidget with a custom drawing function
 	private native void native_set_size_request(int width, int height);
 	protected native void native_destructor(long widget);
+	protected native void native_measure(long widget, int widthMeasureSpec, int heightMeasureSpec);
+	protected native void native_layout(long widget, int l, int t, int r, int b);
+	protected native void native_requestLayout(long widget);
 
 	// --- stubs
 
@@ -879,7 +891,9 @@ public class View extends Object {
 		return system_ui_visibility;
 	};
 
-	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {}
+	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
+		native_measure(widget, widthMeasureSpec, heightMeasureSpec);
+	}
 
 	public void setPressed(boolean pressed) {
 		System.out.println("calling setPressed on " + this + " with value: " + pressed);
@@ -994,14 +1008,93 @@ public class View extends Object {
 
 	public void setOnHoverListener(OnHoverListener listener) {}
 
-	public final void measure (int widthMeasureSpec, int heightMeasureSpec) {}
+	public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
+		onMeasure(widthMeasureSpec, heightMeasureSpec);
+	}
+
+	public final int getMeasuredState() {
+		return 0;
+	}
+
+	public static int combineMeasuredStates(int curState, int newState) {
+		return curState | newState;
+	}
+
+	protected int getSuggestedMinimumHeight() {
+		return 50;
+	}
+	protected int getSuggestedMinimumWidth() {
+		return 100;
+	}
+
+	/**
+	 * Utility to reconcile a desired size and state, with constraints imposed
+	 * by a MeasureSpec.  Will take the desired size, unless a different size
+	 * is imposed by the constraints.  The returned value is a compound integer,
+	 * with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and
+	 * optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the resulting
+	 * size is smaller than the size the view wants to be.
+	 *
+	 * @param size How big the view wants to be
+	 * @param measureSpec Constraints imposed by the parent
+	 * @return Size information bit mask as defined by
+	 * {@link #MEASURED_SIZE_MASK} and {@link #MEASURED_STATE_TOO_SMALL}.
+	 */
+	public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
+		int result = size;
+		int specMode = MeasureSpec.getMode(measureSpec);
+		int specSize =  MeasureSpec.getSize(measureSpec);
+		switch (specMode) {
+		case MeasureSpec.UNSPECIFIED:
+			result = size;
+			break;
+		case MeasureSpec.AT_MOST:
+			if (specSize < size) {
+				result = specSize | MEASURED_STATE_TOO_SMALL;
+			} else {
+				result = size;
+			}
+			break;
+		case MeasureSpec.EXACTLY:
+			result = specSize;
+			break;
+		}
+		return result | (childMeasuredState&MEASURED_STATE_MASK);
+	}
 
 	public final int getMeasuredWidth() {
-		return getWidth();
+		return this.measuredWidth & MEASURED_SIZE_MASK;
 	}
 
 	public final int getMeasuredHeight() {
-		return getHeight();
+		return this.measuredHeight & MEASURED_SIZE_MASK;
+	}
+
+	protected void onLayout(boolean changed, int l, int t, int r, int b) {}
+
+	public void layout(int l, int t, int r, int b) {
+		this.left = l;
+		this.top = t;
+		this.right = r;
+		this.bottom = b;
+		native_layout(widget, l, t, r, b);
+	}
+
+	public int getLeft() {
+		return left;
+	}
+	public int getTop() {
+		return top;
+	}
+	public int getRight() {
+		return right;
+	}
+	public int getBottom() {
+		return bottom;
+	}
+
+	public void offsetTopAndBottom(int offset) {
+		layout(left, top + offset, right, bottom + offset);
 	}
 
 	public void setBackgroundDrawable(Drawable backgroundDrawable) {}
@@ -1016,7 +1109,9 @@ public class View extends Object {
 
 	public boolean removeCallbacks(Runnable action) {return false;}
 
-	public void requestLayout() {};
+	public void requestLayout() {
+		native_requestLayout(widget);
+	};
 
 	public void setOverScrollMode(int mode) {}
 
@@ -1024,12 +1119,12 @@ public class View extends Object {
 
 	public boolean postDelayed(Runnable action, long delayMillis) {
 		new Handler(Looper.getMainLooper()).postDelayed(action, delayMillis);
-        return true;
-    }
+		return true;
+	}
 
 	public boolean post(Runnable action) {
 		new Handler(Looper.getMainLooper()).post(action);
-        return true;
+		return true;
 	}
 
 	public void setSaveFromParentEnabled(boolean enabled) {}