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

interface/wx/defs.h

@@ -546,9 +546,14 @@ enum wxBackgroundStyle
     /* this style is deprecated and doesn't do anything, don't use */
     wxBG_STYLE_COLOUR,
 
-    /* this is a Mac-only style, don't use in portable code */
-    wxBG_STYLE_TRANSPARENT,
+    /**
+        Indicates that the window background is not erased, letting the parent
+        window show through.
 
+        Currently this style is only supported in wxOSX and wxGTK with
+        compositing available, see wxWindow::IsTransparentBackgroundSupported().
+     */
+    wxBG_STYLE_TRANSPARENT,
 };