omega/rect.py: more clarity in complex layout logic
Super messy work-in-progress to improve layout yet again.
Tighten up design a bit and remove lots of dead code.
Move HBox to new layout and fix a positioning bug
omega/latex.py: move to new layout system
Comment/stub layout containers that I'm ignoring for now.

I'll worry about them later.
omega/layout.py: fix linear box layout!

We were exporting the results incorrectly.
omega/rect.py: fix up layout of AbsoluteFieldOverlay boxes

I think that what I was doing before didn't make the most sense. The
boxes now get a bit less padding so they benefit from setting the
default alignment parameters to not-quite-zero.