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/gtk/bitmap.h

@@ -14,8 +14,6 @@ typedef struct _cairo cairo_t;
 typedef struct _cairo_surface cairo_surface_t;
 #endif
 typedef struct _GdkPixbuf GdkPixbuf;
-class WXDLLIMPEXP_FWD_CORE wxPixelDataBase;
-class WXDLLIMPEXP_FWD_CORE wxCursor;
 
 //-----------------------------------------------------------------------------
 // wxMask