CMake: Refactor CMake build (#5032)

* CMake: Refactor build to multiple libraries

- Refactor CMake build system by creating separate libraries for
  different components
- Create interface libraries for most dependencies and add 3rdparty::*
  ALIAS targets for ease of use and use them to try specifying correct
  dependencies for each target
- Prefer 3rdparty:: ALIAS when linking dependencies
- Exclude xxHash subdirectory from ALL build target
- Add USE_SYSTEM_ZLIB option to select between using included ZLib and
  the ZLib in CMake search path

* Add cstring include to Log.cpp

* CMake: Add 3rdparty::glew interface target

* Add Visual Studio CMakeSettings.json to gitignore

* CMake: Move building and finding LLVM to 3rdparty/llvm.cmake script

- LLVM is now built under 3rdparty/ directory in the binary directory

* CMake: Move finding Qt5 to 3rdparty/qt5.cmake script

- Script has to be included in rpcs3/CMakeLists.txt because it defines
  Qt5::moc target which isn't available in that folder if it is
  included in 3rdparty directory
- Set AUTOMOC and AUTOUIC properties for targets requiring them (rpcs3
  and rpcs3_ui) instead of setting CMAKE_AUTOMOC and CMAKE_AUTOUIC so
  those properties are not defined for all targets under rpcs3 dir

* CMake: Remove redundant code from rpcs3/CMakeLists.txt

* CMake: Add BUILD_LLVM_SUBMODULE option instead of hardcoded check

- Add BUILD_LLVM_SUBMODULE option (defaults to ON) to allow controlling
  usage of the LLVM submodule.
- Move option definitions to root CMakeLists

* CMake: Remove separate Emu subtargets

- Based on discussion in pull request #5032, I decided to combine
  subtargets under Emu folder back to a single rpcs3_emu target

* CMake: Remove utilities, loader and crypto targets: merge them to Emu

- Removed separate targets and merged them into rpcs3_emu target as
  recommended in pull request (#5032) conversations. Separating targets
  probably later in a separate pull request

* Fix relative includes in pad_thread.cpp

* Fix Travis-CI cloning all submodules needlessly

.gitignore

@@ -59,6 +59,7 @@
 *.VC.*
 *.vcxproj.user
 enc_temp_folder/*
+CMakeSettings.json
 
 # Ignore other system generated files
 x64/*
@@ -85,3 +86,6 @@ qrc_*.cpp
 rpcs3_automoc.cpp
 ui_*.h
 rpcs3/rpcs3_autogen/*
+
+# QtCreator
+CMakeLists.txt.user