Update just the colour, not font, in wxOSX SetForegroundColour()

Calling wxWindow::SetForegroundColour() also changed the window font,
which was unexpected and unnecessary.

Add a separate peer SetForegroundColour() method and implement it to
change just the colour in Cocoa version (and leave unimplemented, just
as it was before, for iOS) and use it in wxWindow to avoid the font
change.

include/wx/osx/window.h

@@ -342,7 +342,6 @@ protected:
     bool                MacHasScrollBarCorner() const;
     void                MacCreateScrollBars( long style ) ;
     void                MacRepositionScrollBars() ;
-    void                MacUpdateControlFont() ;
 
     // implement the base class pure virtuals
     virtual void DoGetTextExtent(const wxString& string,