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.

include/wx/nativewin.h

@@ -38,8 +38,6 @@
     typedef HWND wxNativeContainerWindowHandle;
     typedef HWND wxNativeWindowHandle;
 #elif defined(__WXGTK__)
-    #include <gtk/gtk.h>
-
     // GdkNativeWindow is guint32 under GDK/X11 and gpointer under GDK/WIN32
     #ifdef __UNIX__
         typedef unsigned long wxNativeContainerWindowId;