Expand the RelationshipSourceCollection to return more information

[Brezak]

Objective

While redoing #18058 I needed RelationshipSourceCollection (henceforth referred to as the Trait) to implement clear so I added it.

Solution

Add the clear method to the Trait.
Also make add and remove report if they succeeded.

Testing

Eyeballs

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Showcase

The RelationshipSourceCollection trait now reports if adding or removing an entity from it was successful.
It also not contains the clear method so you can easily clear the collection in generic contexts.

Changes

EDITED by Alice: We should get this into 0.16, so no migration guide needed.

The RelationshipSourceCollection methods add and remove now need to return a boolean indicating if they were successful (adding a entity to a set that already contains it counts as failure). Additionally the clear method has been added to support clearing the collection in generic contexts.

[Brezak]

I've also considered adding the shrink_to_fit method to the Trait but I don't know how useful it would be.