Favorites for repository navigation

The repository navigation now has a second tab for favorites:

![bildschirmfoto 2014-09-10 um 15 56 34](https://cloud.githubusercontent.com/assets/7469/4218564/a0b0cc8a-38f2-11e4-99b8-fb928cb29e51.png)

The favorites are stored as annotations per repository root, per user.
Favorites can be added and removed from both, the normal tree and the favorites tree.
The selected tab will be select for all following requests until it is switched manually (cookie).

This PR includes smaller refactorings for the tree javascript as well as minor fixes such as the portlet height and scrolling to the currently selected node.

@phgross please take a look :wink: