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)

The concern I have with mixing predefined and translated strings with an option to enter free-form text is that that it becomes less clear what format the free-form text can take. Should it be in title case or initial caps, like the predefined strings? Can you enter strings in your native language? You and I know the answer to those questions is "no, if you're entering free-form text, it's the raw tag value and should follow those conventions", but many contributors will not.

`"indexed": true` -- I don't think this was ever used. It can be removed.