Remove add_children uses

[luca-della-vedova]

New feature implementation

Implemented feature

Remove all uses of add_children throughout the codebase.

Implementation description

In preparation for the upgrade to bevy 0.11, the add_children function was removed. This PR refactors the code to remove all uses to make migration easier without breaking the code (yet!). Two main cases were encountered:

• Cases in which the return value was actually ignored, a simple add_children -> with_children change works.
• Cases in which the return value was used, usually to spawn entities and return their id, remove the add_children, spawn all the entities and manually add them through either add_child or set_parent, depending on the case.

[luca-della-vedova]

At this point it's probably easier to just work on #180 instead, closing this