Add subtitles 

Adds a box for subtitles in the lower left corner of the screen, wrapping alerts and status, and makes them more visible with either a black outline or a translucent background. 

One problem is that, even though the VO doesn't overlap, the `playSubtitle` event fires on the sound init, so that there will be multiple subtitles going off at once. I'll look at that in the core.

@BrettASwift @kadst43