Ability to rename custom properties

Resolve the issue #772 "Can't rename custom properties in Properties window"

A button is added to the interface in order to be able to rename the custom properties, and only custom properties. Here a screenshot of the changes in the GUI 
![capture d ecran 2014-10-17 a 12 30 15](https://cloud.githubusercontent.com/assets/2650571/4677531/e2194f04-55e8-11e4-82ae-b23ea0c78cbc.png)

The "pen" button is the rename property icon.

(sorry for the second pull-request :s)