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/gtk/private.h

@@ -11,10 +11,9 @@
 #ifndef _WX_GTK_PRIVATE_H_
 #define _WX_GTK_PRIVATE_H_
 
-#include <gtk/gtk.h>
+#include "wx/gtk/private/wrapgtk.h"
 
 #include "wx/gtk/private/string.h"
-#include "wx/gtk/private/gtk2-compat.h"
 
 #ifndef G_VALUE_INIT
     // introduced in GLib 2.30

include/wx/gtk/private/addremovectrl.h

@@ -14,7 +14,7 @@
 #include "wx/bmpbuttn.h"
 #include "wx/toolbar.h"
 
-#include <gtk/gtk.h>
+#include "wx/gtk/private/wrapgtk.h"
 
 // ----------------------------------------------------------------------------
 // wxAddRemoveImpl

include/wx/gtk/private/wrapgtk.h

@@ -0,0 +1,19 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/gtk/private/wrapgtk.h
+// Purpose:     Include gtk/gtk.h without warnings and with compatibility
+// Author:      Vadim Zeitlin
+// Created:     2018-05-20
+// Copyright:   (c) 2018 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_GTK_PRIVATE_WRAPGTK_H_
+#define _WX_GTK_PRIVATE_WRAPGTK_H_
+
+wxGCC_WARNING_SUPPRESS(parentheses)
+#include <gtk/gtk.h>
+wxGCC_WARNING_RESTORE(parentheses)
+
+#include "wx/gtk/private/gtk2-compat.h"
+
+#endif // _WX_GTK_PRIVATE_WRAPGTK_H_

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;