Bump bevy version to 0.8.0

Remove unused bevy features

Fixes #110

Create bevy_event.rs

Allow minimising

Gives an example how to send a bevy event from a button event handler. It took me a while to figure this out so I thought others might find this helpful.

Use full path to Index in macros

This change is copied verbatim from bevy itself. See:
https://github.com/bevyengine/bevy/commit/09a3d8abe062984479bf0e99fcc1508bb722baf6

Fix transparency for type 0 quads

Revert "kayak_core: Improve `Tree` methods"

Also added some additional helper methods and cleaned up some existing
ones

Re-implemented with new depth map (should be more performant)

kayak_font: Add `Grapheme` and `RowCol`

Fix `DownwardIterator`

Fixed issue where DownwardIterator would iterate through siblings once
finished iterating through descendants

Also added tests 🧪

Improved kayak_font