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.

samples/widgets/native.cpp

@@ -143,6 +143,8 @@ protected:
 
 #elif defined(__WXGTK__)
 
+#include <gtk/gtk.h>
+
 class NativeWindow : public wxNativeWindow
 {
 public: