Added support for Get/SetColour in wxFontPickerCtrl

Pull request for http://trac.wxwidgets.org/ticket/11614

I've also taken the liberty of replacing a define with a private member function and replacing an old style C cast with a static_cast.

I've added two virtual methods to wxFontPickerWidgetBase which is used to represent the control that displays the font dialog. They allow the setting and getting of the font colour.

For wxMSW these are implemented through the wxFontData available.
For wxGTK2 these are implemented by using a data member.

A simple tests case is also included.