Fixed #4686. Small description of activity displayed in palette.

Bug: http://bugs.sugarlabs.org/ticket/4686

Used `Gtk.Label` with Pango based ellipsize for wrapping and ellipsizing.

Please see #259 for the previous version of the patch. I've made the entire wrapping and ellipsization using Gtk and Pango, as suggested in the last version of the patch.

_Large Activity Description_
![sugar_large](https://cloud.githubusercontent.com/assets/2565248/2538545/f7a1f684-b5be-11e3-99d8-b60a1cc52dc9.png)

_Small Activity Description_
![sugar_small](https://cloud.githubusercontent.com/assets/2565248/2538556/13d7b46a-b5bf-11e3-8c4a-adfc3bfe5b16.png)