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/print.cpp

@@ -34,7 +34,7 @@
 #include "wx/paper.h"
 #include "wx/modalhook.h"
 
-#include <gtk/gtk.h>
+#include "wx/gtk/private/wrapgtk.h"
 
 #if GTK_CHECK_VERSION(2,14,0)
 #include <gtk/gtkunixprint.h>
@@ -46,7 +46,6 @@
 wxFORCE_LINK_THIS_MODULE(gtk_print)
 
 #include "wx/gtk/private/object.h"
-#include "wx/gtk/private/gtk2-compat.h"
 
 // Useful to convert angles from degrees to radians.
 static const double DEG2RAD  = M_PI / 180.0;