Feature/#841 open folder menu

Implements https://github.com/spritebuilder/SpriteBuilder/issues/841

Looking like this:

![bildschirmfoto 2014-11-26 um 22 50 07](https://cloud.githubusercontent.com/assets/5259653/5209410/a5f952c6-75be-11e4-9028-e18cb53699f1.png)