[desktop] More qt_common reorganization (#3916)

Ported from QML branch.

Main "big" change is that EmuThread is now a shared state in QtCommon,
not individually managed/passed around by GRenderWindow and MainWindow.

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3916
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/common/settings.h

@@ -863,6 +863,8 @@ bool IsFastmemEnabled();
 void SetNceEnabled(bool is_64bit);
 bool IsNceEnabled();
 
+bool IsOpenGL();
+
 bool IsDockedMode();
 
 float Volume();