Added version switching

Added functionality to switch between different data folders. Custom data folders must have their names started with "Data_"
Also added a backup when user wants to re-download tree assets.
TODO: new icon is needed for this new menu item, as I don't know where you took them :)
![image](https://cloud.githubusercontent.com/assets/1394349/5360603/d2b58006-7fd6-11e4-8e90-351c90bc6355.png)