generic component propagation

[robtfm]

Objective

add functionality to allow propagating components to children. requested originally for RenderLayers but can be useful more generally.

Solution

• add HierarchyPropagatePlugin<C, F=()> which schedules systems to propagate components through entities matching F
• add Propagate<C: Component + Clone + PartialEq> which will cause C to be added to all children
  more niche features:
• add PropagateStop<C> which stops the propagation at this entity
• add PropagateOver<C> which allows the propagation to continue to children, but doesn't add/remove/modify a C on this entity itself

Testing

see tests inline

Notes

• could happily be an out-of-repo plugin
• not sure where it lives: ideally it would be in bevy_ecs but it requires a Plugin so I put it in bevy_app, doesn't really belong there though.
• i'm not totally up-to-date on triggers and observers so possibly this could be done more cleanly, would be very happy to take review comments
• perf: this is pretty cheap except for update_reparented which has to check the parent of every moved entity. since the entirety is opt-in i think it's acceptable but i could possibly use (Changed<Children>, With<Inherited<C>>) instead if it's a concern