Fixed ticket 4686.

Small description of activity shown in palette. See the companion PR as well : https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/131

#### Large Description
![sugar_long](https://cloud.githubusercontent.com/assets/2565248/2595325/6c9455e8-ba99-11e3-9527-145976d5d94f.png)

#### Small Description
![sugar_short](https://cloud.githubusercontent.com/assets/2565248/2595341/9a69dc40-ba99-11e3-8da9-a403b17b1c4b.png)