Evolution/2385/extract button

for http://forge.applis-bretagne.fr/issues/2385

For the second part of the issue:

> De plus, modifier la couleur pour que le bouton soit visible (même apparence que le bouton Modifier cette emprise)

I tried to set the panel background to blue gradient, copied from Github button and shifted from green to blue. I don't know if it's a good idea, principally because of the non-respect of the Ext themes (we use the "gray" theme). Please give feedback. I join a screenshot.

![extractor screenshot](https://f.cloud.github.com/assets/1676121/850939/aea74cba-f492-11e2-9dc1-9cc6fb74748c.png)