Overhaul the RA sidebar.

![sidebar](https://cloud.githubusercontent.com/assets/167819/3610300/6a739d6c-0d89-11e4-9dfc-b580d795f9c3.png)

This generalizes the TD ingame UI code, and uses it to implement a new command-panel design for RA.
This cuts down on the code duplication, and gives RA a much-needed facelift.

Closes #2394, #2385, #4980, #5108, #5543, and probably others.