Add a class derived from GtkImage to support HiDPI bitmaps

And use it to get HiDPI support wherever we use GtkImage. This extends and
consolidates support for custom drawing of images which has already been added
somewhat redundantly in several places.

src/gtk/button.cpp

@@ -19,6 +19,7 @@
 
 #include "wx/gtk/private.h"
 #include "wx/gtk/private/list.h"
+#include "wx/gtk/private/image.h"
 
 // ----------------------------------------------------------------------------
 // GTK callbacks
@@ -98,7 +99,7 @@ bool wxButton::Create(wxWindow *parent,
     {
         m_widget = gtk_button_new();
 
-        GtkWidget *image = gtk_image_new();
+        GtkWidget* image = wxGtkImage::New(this);
         gtk_widget_show(image);
         gtk_container_add(GTK_CONTAINER(m_widget), image);
     }