Ignore "Keep plugin windows on top" setting when on wayland (#7901)

Fix a crash that occurs when running under Wayland and when loading a VST
plugin with the option "Keep plugin windows on top when not embedded"
enabled.

include/GuiApplication.h

@@ -55,6 +55,7 @@ public:
 
 	static GuiApplication* instance();
 	static void sigintHandler(int);
+	static bool isWayland();
 #ifdef LMMS_BUILD_WIN32
 	static QFont getWin32SystemFont();
 #endif