This also probably fixes bugs with wrong z-indexing for fresh entities

Entities are only selected after creation if requested - this was
taking a lot of time, and was pointless. The tick-drawing function
doesn't read calculated styles as much now, but it still forces
a reflow when setting the font, weirdly.

Previously, every call to updateSizes() would update every entity. Now, entities
can be marked as dirty, and updateSizes can be called such that it only affects
dirty entities. I changed a few calls to updateSizes to take advantage of this,
but others can probably be optimized too