Add the possibility of display a pixbuf in the palette.

This is needed to implement preview of images ([1] and [2]) in clipboard buttons.

[1] http://wiki.sugarlabs.org/go/Design_Team/Specifications/Clipboard#Previews
[2] http://wiki.sugarlabs.org/go/File:Frame-05.jpeg

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>