Move QT_X11_NO_NATIVE_MENUBAR to code (LMMS#4818)

Set Qt::AA_DontUseNativeMenuBar
Remove QT_X11_NO_NATIVE_MENUBAR from desktop launcher

cmake/linux/lmms.desktop

@@ -8,7 +8,7 @@ Comment=Music sequencer and synthesizer
 Comment[ca]=Producció fàcil de música per a tothom!
 Comment[fr]=Production facile de musique pour tout le monde !
 Icon=lmms
-Exec=env QT_X11_NO_NATIVE_MENUBAR=1 lmms %f
+Exec=lmms %f
 Terminal=false
 Type=Application
 Categories=Qt;AudioVideo;Audio;Midi;

src/core/main.cpp

@@ -252,6 +252,10 @@ int main( int argc, char * * argv )
 		return EXIT_FAILURE;
 	}	
 #endif
+#ifdef LMMS_BUILD_LINUX
+	// don't let OS steal the menu bar. FIXME: only effective on Qt4
+	QCoreApplication::setAttribute( Qt::AA_DontUseNativeMenuBar );
+#endif
 #if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
 	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 #endif