Restore bevy_feathers features after merge with upstream

[villor]

bevyengine#20158

Looks like the recent sync with bevy main caused us to lose some feathers stuff

• Added the (next-gen-scenes-specific) pane examples back to the feathers example
• Migrated tool_button to use EntityCursor
• Fixed AlphaPattern not setting the MaterialNode handle properly
  • This was probably due to the fact that spawn_scene does not insert the whole bundle at once yet
  • Now uses #[require] and a component hook to initialize it in a way that works with spawn_scene - a nice plus was this allowed us to get rid of the extra plugin and resource
• Cleaned up some unused imports

Back in business:

[viridia]

Haha, now I don't have to work on this!

[cart]

Oops! Much appreciated!