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

interface/wx/panel.h

@@ -124,21 +124,6 @@ public:
     */
     void OnSysColourChanged(wxSysColourChangedEvent& event);
 
-    /**
-        Set the background bitmap for this panel.
-
-        If @a bmp is a valid bitmap, this bitmap will be tiled over the panel
-        background and show through any of its transparent children. Passing an
-        invalid bitmap reverts to the default background appearance.
-
-        Notice that you must not prevent the base class EVT_ERASE_BACKGROUND
-        handler from running (i.e. not to handle this event yourself) for this
-        to work.
-
-        @since 2.9.2
-    */
-    void SetBackgroundBitmap(const wxBitmap& bmp);
-
     /**
         Overrides wxWindow::SetFocus().