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.

include/wx/gtk/private/image.h

@@ -0,0 +1,33 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/gtk/private/image.h
+// Author:      Paul Cornett
+// Copyright:   (c) 2020 Paul Cornett
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+// Class that can be used in place of GtkImage, to allow drawing of alternate
+// bitmaps, such as HiDPI or disabled.
+
+class wxGtkImage: GtkImage
+{
+public:
+    struct BitmapProvider
+    {
+        virtual ~BitmapProvider() { }
+        virtual wxBitmap Get() const = 0;
+        virtual void Set(const wxBitmap&) { }
+    };
+
+    static GType Type();
+    static GtkWidget* New(BitmapProvider* provider);
+    static GtkWidget* New(wxWindow* win = NULL);
+    void Set(const wxBitmap& bitmap);
+
+    BitmapProvider* m_provider;
+
+    wxDECLARE_NO_COPY_CLASS(wxGtkImage);
+    wxGtkImage() wxMEMBER_DELETE;
+    ~wxGtkImage() wxMEMBER_DELETE;
+};
+
+#define WX_GTK_IMAGE(obj) G_TYPE_CHECK_INSTANCE_CAST(obj, wxGtkImage::Type(), wxGtkImage)