Update to bevy 0.15 (#166)

* feat: main code compiles (untested)

* feat: basic_sprite example works as expected

* mark: example basic_ui works

* mark: every_option example works
except for the border, which was just removed

* mark: example font_per_widget works

* mark: image_background example works

* marK: multiple_sprites example works

* marK: placeholder example works

* marK: readonly example works

* marK: readonly example works

* mark: final example works!

* feat: updated module structure to use a prelude

* feat: fixed all warnings

* fix: tests pass

* refactor: `CosmicBuffer` -> `CosmicEditBuffer` as `bevy::text::CosmicBuffer` already exists

* todo: remove cosmic source component

* refactor: Removed `CosmicSource`

* feat: Removed old `CosmicWidgetSize` component in favour of an implementation-specific getter world query

* feat: half the examples compiling and working
Except for this annoying bug with the ui looking weird

* mark: only every_option example to go

* feat: All examples ported over

* refactor: Renamed `ScrollDisabled` to `ScrollEnabled`

* fix: cargo doc returns no warnings

* refactor: Using bevy builtin `SwashCache`

* fix: documentation is now more consistent

* fix: documentation and privacy are now much better

* fix: wasm compiles again

* refactor: utils is now private module

* doc: minor adjustments

* fix: utils module now public

* doc: documented `print_editor_sizes`

* fmt

* doc: todo

* todo: move all target-specific code into render_targets module

* todo: clean up lots of mathy code todo with render boxes

* wip: trying to debug annoying UI start glitch

* feat: fixed final UI bug

* fmt