Reuse the already created node in `PortraitManager.add_portrait` and avoid removing the portrait in `CharacterJoin`

[Crystalwarrior]

PortraitManager: do not remove portrait of self if:

• it detects we already have a portrait on the scene
• if remove_other_portraits is set

To remove portrait of self, remove_portrait must explicitly be called instead. For join event, you can clear all portraits if you tick "remove_other_portraits" and do not provide a character.

[Crystalwarrior]

For now I would only think about what can happen if you add a portrait that is already on scene. It'll not call the change_portrait method, so no portrait_changed signal will be emmited. Maybe we can call the method directly if the portrait node exists? No idea

think so yea, let's do that in a separate pr tho

[AnidemDex]

Two things in a single PR, is agains the contribution but whatever. Thank you!