Add link to opencs documentation on readthedocs, available through

context menu.
The documentation opens in default browser.

There are 3 contexts:
- global: opens the OpenMW CS User Manual main page
- when a record is selected: opens the "Tables" page
- when the filter field is selected: opens the "Record Filters" page

There is also a link to the OpenCS tutorial in the help menu.

components/misc/helpviewer.cpp

@@ -0,0 +1,12 @@
+#include "helpviewer.hpp"
+
+#include <QString>
+#include <QDesktopServices>
+#include <QUrl>
+
+void Misc::HelpViewer::openHelp(const char* url)
+{
+    QString link {OPENMW_DOC_BASEURL};
+    link.append(url);
+    QDesktopServices::openUrl(QUrl(link));
+}