* Minor doc, style, and perf updates to Navigator/Routes

These are minor fixes I ended up making while working on a larger
project that never went anywhere.

- Used a ColoredBox instead of a DecoratedBox for ModalBarrier
  (probably a trivial memory/perf win).

- A bunch of Navigator documentation fixes around when things rebuild.

- Mark routes dirty when the Navigator has a dependency change. I
  cannot find a way to test this because as far as I can tell it makes
  no actual difference to when things rebuild because whenever the
  Navigator rebuilds the Overlay rebuilds and whenever that happens
  every OverlayEntry rebuilds, but in theory that's not guaranteed so
  this is sort of a correctness fix. It may even be a perf loss. We do
  something similar in didUpdateWidget already. I could be convinced
  to maybe remove these...

- Make ModalRoute.filter public like everything else.

- Made ModalRoute update its barrier when it gets an update, in case
  e.g. the modal barrier depends on inherited widgets via the
  navigator context. Again, not sure of any way to detect this, it
  might actually be moot, but it seems to be the technically correct
  solution?

- Minor style fixes.

All in all I couldn't figure out a way to test any of this (I wrote
multiple large tests but it turns out they all already pass on master
and are effectively redundant with existing tests).

* Remove extraneous blank line