Move AddObserver and observe from bevy_ui_widgets into bevy_ecs

[dlom]

Objective

• Addressing the top comment in the file:

TODO: This probably doesn't belong in bevy_ui_widgets, but I am not sure where it should go.

Solution

• I moved the module from bevy_ui_widgets into bevy_ecs (under the observer module)
• This seems like the most logical landing spot

Testing

• cargo run -p -- compile
• cargo run -p -- test

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[alice-i-cecile]

This is something that we've discussed, and probably won't do. It's more likely that bsn lands, or we allow observers to be defined using relations.