Improve the usability and transitions of toolbar

Change the toolbar transitions, hover actions, and push states to
improve usability and feel.

- Animate from the right, so that all buttons enter as a group, so
  as to give them all equal "weight"
- Colorize the pushed buttons even when not hovering so it's easier
  to see which are active
- Further emphasize pushed state by "docking" them to the right
- Rework curves, delays, and durations for great beauty
- Show hidden buttons when hovering anywhere, not just the top

Close #1268
Merge pull request #1316 from hypothesis/toolbar-usability

Improve the usability and transitions of toolbar