WIP: Import selection and editing code from Masonry

[nicoburns]

This is a little earlier than planned as masonry's dependency on vello was making this awkward to use from masonry (when I am not necessary depending on the same version of vello).

• Builds upon #76 which should be merged first.

Changes made

• Add dependencies on xi-unicode, unicode_segmentation, dpi, and winit
• Import text2 module from masonry
• Import type ArcStr = Arc<str> from masonry
• Import event types from masonry
• Remove vello dependency (drawing is left up to the user)
• Fix imports of other dependencies
• Updated to work with latest parley API

Todo

• Feature flag editing feature
• Move selection out of editor module?
• Simplify event types
• Add a new (non-masonry) way to be notified when text changes in response to an event