Add color picker button

The color picker button is a simple button, which lets the user pick a color. It opens a SelectColorDialog and displays the selected color when finished.

This PR contains a ColorPicker XWT implementation with a default Xwt based backend, which is used if the active backend lacks IColorPickerBackend implementation. The default backend mimics GtkColorButton (https://developer.gnome.org/gtk2/2.24/GtkColorButton.html).

A native Gtk IColorPickerBackend implementation is included, too.