Fix Labels

[mockersf]

found two issues with Labels while trying to use them in a scene:

• they are not registered which caused panic thread 'main' panicked at 'scene contains the unregistered type bevy_core::label::Labels
• a new label was not added to the label_entities hash map so it remained empty

[mockersf]

I didn't touch the TODO: use Changed as the system is running in stage PRE_UPDATE
Should the stage be changed?

[payload]

Note that removing the labels afte entity despawn looks like also broken. Adding tests to this file would be good.

[payload]

I propose this PR as an alternative which fixes some corner cases and tests them. Feel free to take the tests in case you want to take a different approach: #1152

[cart]

Closing in favor of #1152