Step bullets: Add light blue

Fixes #233.

This adds a light blue bullet to step view and step edit and does a few
other things along the way:

 - Replace all bullet color image drawables with shape drawables so we
   don't have 2 sets of images that don't match and are hard to update.
 - Remove some unused drawables (there were dark and light copies of a
   few of them but only one version was ever used).
 - Standardize on violet rather than having it split between violet,
   purple, and pink.