Update editor view

[AnidemDex]

This is a kinda big update and I used this chance to adjust things for the future.

What's new?

• I changed our Tree implementation with a custom/fake one, using a trick with a bunch of containers. It should fix https://github.com/AnidemDex/Blockflow/issues/92 . It also includes a multi-selection thing, so it should fix #106 (but it does nothing for now)
• I updated our Command to include stuff related to the new fake tree. It should fix https://github.com/AnidemDex/Blockflow/issues/164
• I also updated Collection to expose more internal properties, just for debug purposes, using a EditorInspector.

I think it also fix #161 and fix #107 (fix #92 for sure, fix #164)

Since now our command blocks are nodes and not objects (as was before with Tree), users can make their own blocks in custom scenes. I decided to not make the whole block replicate the old Tree look to force me to create a scene that mimics that old TreeItem look

[AnidemDex]

Bugs so far:

• UndoRedo doesn't work
• Because of that, block operations fails

[AnidemDex]

UndoRedo restored splitting runtime and editor behavior