Implement support for markup labels for wxGTK wxButton.

Simply directly set the markup for the GtkLabel used by GtkButton internally.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67066 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/button.h

@@ -67,6 +67,10 @@ protected:
     virtual void DoSetBitmap(const wxBitmap& bitmap, State which);
     virtual void DoSetBitmapPosition(wxDirection dir);
 
+#if wxUSE_MARKUP
+    virtual bool DoSetLabelMarkup(const wxString& markup);
+#endif // wxUSE_MARKUP
+
 private:
     typedef wxButtonBase base_type;
 
@@ -91,6 +95,10 @@ private:
     // show the given bitmap (must be valid)
     void GTKDoShowBitmap(const wxBitmap& bitmap);
 
+    // Return the GtkLabel used by this button.
+    GtkLabel *GTKGetLabel() const;
+
+
     // the bitmaps for the different state of the buttons, all of them may be
     // invalid and the button only shows a bitmap at all if State_Normal bitmap
     // is valid