personalize: reduce include impact of plugins.h

use forward declaration of CPlugins instead of including
plugins.h in personalize.h to reduce the massive impact
of this header file

src/gui/personalize.cpp

@@ -171,6 +171,7 @@
 #include "widget/keychooser.h"
 #include "color.h"
 #include "personalize.h"
+#include "plugins.h"
 
 #include <system/settings.h>