Support option strings for combo fields

For #1259 and #1709
Now combo fields can support a strings object instead of just displaying the keys.  This allows for friendlier (and translatable!) values.

I did not add strings for every preset with a combo field, but I did convert many of the most useful ones in 98119b5.

![screenshot 2014-07-21 16 29 36](https://cloud.githubusercontent.com/assets/38784/3649880/9f41e842-1116-11e4-916d-0b237f27ab49.png)
![screenshot 2014-07-21 16 29 18](https://cloud.githubusercontent.com/assets/38784/3649883/a1249b82-1116-11e4-8f4e-bf3956ae1b14.png)

This is an asynchronous call, so there needs to be some mechanism for triggering updates to the downstream uses of `strings` after it completes.