Add common implementations of wxStaticBitmap icon methods

Instead of defining them, slightly differently, in all the non-MSW
ports, define them once in wxStaticBitmapBase.

No real changes, this is just a simplification.

include/wx/gtk/statbmp.h

@@ -34,18 +34,9 @@ public:
                  long style = 0,
                  const wxString& name = wxASCII_STR(wxStaticBitmapNameStr));
 
-    virtual void SetIcon(const wxIcon& icon) wxOVERRIDE { SetBitmap( icon ); }
     virtual void SetBitmap( const wxBitmap& bitmap ) wxOVERRIDE;
     virtual wxBitmap GetBitmap() const wxOVERRIDE { return m_bitmap; }
 
-    // for compatibility with wxMSW
-    wxIcon GetIcon() const wxOVERRIDE
-    {
-        // don't use wxDynamicCast, icons and bitmaps are really the same thing
-        // in wxGTK
-        return (const wxIcon &)m_bitmap;
-    }
-
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);