Bevy 0.15

[UkoeHB]

TODOs for the migration to Bevy 0.15.

• re-enable directional isolates (need cosmic-text)
• adjust text system ordering (need UiSystem system set updates)
• adjust TextEditor implementation based on introduction of text spans (will need to do a deeper redesign of text localization at a later time)
• add OverflowClipMargin to Dims
  • https://github.com/bevyengine/bevy/pull/15561/files

Future work after the migration:

• look into using built-in picking for interactions
• forward interactions to bevy_cobweb entity events
  • add observers for interactions
  • each observer sends an event to a unified stream
  • unified stream is forwarded into bevy_cobweb entity events in PreUpdate in system after bevy's interaction system
• look into replacing sickle_ui animations with Allow animation clips to animate arbitrary properties. bevyengine/bevy#15282
  • or at least integrate this for animations (e.g. long-running animations unrelated to interaction)
• investigate built-in bevy animation as a replacement for flux interactions
  • Allow animation clips to animate arbitrary properties. bevyengine/bevy#15282
• add scrolling widget now that ScrollPosition was added
• try to add LoadedScene::get() and LoadedScene::get_from_root() that return new LoadedScenes (requires EntityCommands::commands_mut())
• implement cursor controls, including custom user-defined cursors.