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

include/wx/generic/panelg.h

@@ -42,18 +42,7 @@ public:
     )
 #endif // WXWIN_COMPATIBILITY_2_8
 
-protected:
-    virtual void DoSetBackgroundBitmap(const wxBitmap& bmp);
-
 private:
-    // Event handler for erasing the background which is only used when we have
-    // a valid background bitmap.
-    void OnEraseBackground(wxEraseEvent& event);
-
-
-    // The bitmap used for painting the background if valid.
-    wxBitmap m_bitmapBg;
-
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxPanel);
 };