Unify characters into a single repository/provider

[ethanmoffat]

There isn't really any reason to keep MainCharacter in its own repository. It's convenient, but leads to a lot of code such as

if (id == _characterProvider.MainCharacter.ID)
{
...
}
else if (_currentMapStateProvider.Characters.Contains(id))
{
...
}
else
{
...
}

Which is pretty bloated when the operations are usually the same thing.

Instead, it would be better to store the ID of the main player in the PlayerInfoProvider and put all characters and their renderers in the same collection, without differentiating between "main player" and other logged-in players. Packets that are handled specifically for the main player can use the property in PlayerInfoProvider to find the right character.