Editor - Add CollapsiblePanel

Adds `UI.CollapsiblePanel` and changes the top-level contents of the SidePanel to use this class. The `UI.CollapsiblePanel` allows the user to hide unnecessary information by collapsing panels, making for a more minimalist environment that is conducive to productivity. :smile_cat:

![image](https://cloud.githubusercontent.com/assets/1474978/2692529/9990d752-c3a3-11e3-8741-473576d863c7.png)