Add wxPanel::SetBackgroundBitmap().

This method provides a simple way to set a background bitmap without defining
an EVT_ERASE_BACKGROUND handler and, more importantly, one that works
correctly in wxMSW for a window with children as it paints the background of
transparent children too.

Add a test of this method to the erase sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67253 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -464,6 +464,7 @@ All (GUI):
 - Added wxUIActionSimulator (Steven Lamerton, GSoC 2010 project).
 - wxAUI: support auto-orientable toolbars (wsu).
 - wxAUI: add support for icons in pane title bars (triton).
+- Added wxPanel::SetBackgroundBitmap().
 - Added wxDataViewCtrl::Set/GetCurrentItem().
 - Added possibility to disable individual wxDataViewCtrl items (Neno Ganchev).
 - wxHTML: render in RTL order inside RTL window (Richard Bullington-McGuire).