Implement wxBG_STYLE_TRANSPARENT support for wxGTK.

Use composited windows if supported by GTK+ for wxWindows with this background
style.

Also add wxWindow::IsTransparentBackgroundSupported() and show how to use it
in the sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70569 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/window.h

@@ -81,6 +81,7 @@ public:
     virtual bool SetFont( const wxFont &font );
 
     virtual bool SetBackgroundStyle(wxBackgroundStyle style) ;
+    virtual bool IsTransparentBackgroundSupported(wxString* reason = NULL) const;
 
     virtual int GetCharHeight() const;
     virtual int GetCharWidth() const;
@@ -399,7 +400,6 @@ private:
     bool DoScrollByUnits(ScrollDir dir, ScrollUnit unit, int units);
     virtual void AddChildGTK(wxWindowGTK* child);
 
-
     DECLARE_DYNAMIC_CLASS(wxWindowGTK)
     wxDECLARE_NO_COPY_CLASS(wxWindowGTK);
 };