Handling for text centering

:warning: WIP :warning:

This PR will get text centering working with the simple scheme outlined at https://github.com/shoes/shoes4/issues/522#issuecomment-44852358:

Primary algorithm:
- [x] Centered text consumes the full containing width of the parent element it is placed in
- [x] If text isn't the first thing on a given line, bump down to a new line then consume it entirely.

Other tasks to make sure about:
- [x] Margins are respected on centered text (as respected as any other margins. See #750)
- [x] Check works as expected in nested slots
- [x] Check behavior with explicit width text blocks (should use that width and center in it, allow flowing afterward though... second-guessing whether these should bump down or not... hrm)
- [x] ~~Improve fitting tests as possible (noticed that they're a bit terse and cryptic coming back to them now, and don't express things that can actually exercise what I want about the centering)~~ (Will handle separately from this PR)
- [ ] Test with @PragTob presentation Shoes app