Remove redundant forward decls and headers from wx/*/bitmap.h

No real changes, just don't redeclare the classes already forward
declared in wx/bitmap.h in the files included (only) from it and don't
include redundant headers.

Also forward declare wxCursor and wxPixelDataBase in wx/bitmap.h itself
for consistency, as many (even though not all) headers use them.

Do not forward declare wxControl in wx/{msw,osx}/bitmap.h, however,
there is no reason to do it in this header.

include/wx/gtk1/bitmap.h

@@ -10,14 +10,9 @@
 #ifndef __GTKBITMAPH__
 #define __GTKBITMAPH__
 
-#include "wx/defs.h"
-#include "wx/object.h"
-#include "wx/string.h"
 #include "wx/palette.h"
 #include "wx/gdiobj.h"
 
-class WXDLLIMPEXP_FWD_CORE wxPixelDataBase;
-
 //-----------------------------------------------------------------------------
 // classes
 //-----------------------------------------------------------------------------