Show preview in the clipboard button palette for images

This was designed long time ago ([1] and [2]) but was not possible with
the old palettes. Now with gtk3, we can implement it.

[1] http://wiki.sugarlabs.org/go/Design_Team/Specifications/Clipboard#Previews
[2] http://wiki.sugarlabs.org/go/File:Frame-05.jpeg

The pull request https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/25 is needed by this feature