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/panel.h

@@ -52,26 +52,11 @@ public:
                 const wxString& name = wxPanelNameStr);
 
 
-    // Use the given bitmap to tile the background of this panel. This bitmap
-    // will show through any transparent children.
-    //
-    // 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.
-    void SetBackgroundBitmap(const wxBitmap& bmp)
-    {
-        DoSetBackgroundBitmap(bmp);
-    }
-
-
     // implementation from now on
     // --------------------------
 
     virtual void InitDialog();
 
-protected:
-    virtual void DoSetBackgroundBitmap(const wxBitmap& bmp) = 0;
-
 private:
     wxDECLARE_NO_COPY_CLASS(wxPanelBase);
 };