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/msw/panel.h

@@ -20,7 +20,7 @@ class WXDLLIMPEXP_FWD_CORE wxBrush;
 class WXDLLIMPEXP_CORE wxPanel : public wxPanelBase
 {
 public:
-    wxPanel() { Init(); }
+    wxPanel() { }
 
     wxPanel(wxWindow *parent,
             wxWindowID winid = wxID_ANY,
@@ -29,13 +29,9 @@ public:
             long style = wxTAB_TRAVERSAL | wxNO_BORDER,
             const wxString& name = wxPanelNameStr)
     {
-        Init();
-
         Create(parent, winid, pos, size, style, name);
     }
 
-    virtual ~wxPanel();
-
     // This is overridden for MSW to return true for all panels that are child
     // of a window with themed background (such as wxNotebook) which should
     // show through the child panels.
@@ -54,18 +50,7 @@ public:
     )
 #endif // WXWIN_COMPATIBILITY_2_8
 
-protected:
-    void Init()
-    {
-        m_backgroundBrush = NULL;
-    }
-
-    virtual void DoSetBackgroundBitmap(const wxBitmap& bmp);
-    virtual WXHBRUSH MSWGetCustomBgBrush();
-
 private:
-    wxBrush *m_backgroundBrush;
-
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxPanel);
 };