margin: auto for positioned elements

[hrj]

For positioned elements, the auto styles need to take care of top, bottom, left, right properties as well. Moreover they might need to be processed at a different time in the layout sequence.

Note: I added an if not positioned condition around applyAutoStyles in https://github.com/UprootLabs/gngr/commit/dff3aa39da012031fc15d7e2a4be1c35e274f1a0 and saw 9 progressions.