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/core/private.h

@@ -276,6 +276,7 @@ public :
 
     virtual void        SetBackgroundColour( const wxColour& col ) = 0;
     virtual bool        SetBackgroundStyle(wxBackgroundStyle style) = 0;
+    virtual void        SetForegroundColour( const wxColour& col ) = 0;
 
     // all coordinates in native parent widget relative coordinates
     virtual void        GetContentArea( int &left , int &top , int &width , int &height ) const = 0;