Move SetBackgroundBitmap() from wxPanel to new wxCustomBackgroundWindow.

wxCustomBackgroundWindow is a new class allowing to set a custom bitmap for
the background of any window. The relevant code was mostly moved from wxPanel
to which it was added only recently (before 2.9.2) making it unnecessary to
preserve compatibility.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69378 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -444,6 +444,11 @@ Major new features in this release
 2.9.3:
 ------
 
+INCOMPATIBLE CHANGES SINCE 2.9.2
+
+- wxPanel::SetBackgroundBitmap() was removed, derive your class from
+  wxCustomBackgroundWindow and use its method with the same name instead.
+
 All:
 
 - Fix parsing of negated long options in wxCmdLineParser (roed_bis).