Replace MultiplayerRoomComposite with local bindings

[smoogipoo]

This is the start of trying to make the multiplayer / online play structure more understandable, and safe in light of the discovery of unsafe bindable bindings that are killing tests and have also been seen in the wild.

A few important parts to look out for:

Scope of changes

I've tried to make as little changes as possible, but...

• I've continued to bind to RoomUpdated in cases where that could be replaced with a (not yet existing) HostChanged event.
• #nullable disable has been removed from every touched class.

Scheduling semantics

• OnRoomUpdated()/OnRoomLoadRequested() use Scheduler.AddOnce(). The semantics have not changed.
• UserJoined()/UserKicked()/UserLeft() uses Scheduler.Add(). These now use Schedule() which applies the transform time but I don't believe this to be significant going by the originating commit.
• All other events use Schedule(). The semantics have not changed.

[peppy]

As mentioned IRL, I'm kinda against having raw IBindable<> things being cached like this. I know this is an existing usage, but hoping we can refactor as a future pass. For instance, putting the bindables inside a MultiplayerState would resolve the issue for me.

I just really dislike caching of bindables like this (and tracking down where the bindables are cached). It took me quite some minutes to figure out that you weren't adding a new cached bindable here and that it was already being provided in RoomSubScreen.

[smoogipoo]

Completely agree. The path I'm taking right now is to completely remove bindable caching either directly or ferried via another object, and seeing if a case comes up where it breaks down and the current structure makes more sense (hopefully not).

Basically, I don't know how things will end up yet because I haven't gone the full way 😛 It's entirely possible this PR may be reverted at some point if things don't pan out well.