Avoid many -Wparentheses warnings from gtk/gtk.h with gcc8

Add wx/gtk/private/wrapgtk.h wrapping gtk/gtk.h in pragmas disabling
these warnings and include it everywhere instead of directly including
gtk/gtk.h.

Also include wx/gtk/private/gtk2-compat.h from this wrapper header as it
was included by 90% of the files including gtk/gtk.h itself and it seems
to be better and simpler to just always include it.

src/gtk/bitmap.cpp

@@ -23,8 +23,6 @@
 #include "wx/gtk/private/object.h"
 #include "wx/gtk/private.h"
 
-#include <gtk/gtk.h>
-
 GdkWindow* wxGetTopLevelGDK();
 
 #ifndef __WXGTK3__