Support token locking to disable interactions by other players

[samcf]

The host may want to prevent players from interacting with non-player tokens. This is especially important to prevent players from accidentally selecting tokens through a scene mask.

1. New control on the token toolbar.
2. By default, all tokens are locked.
3. Tokens are implicitly unlocked if they become player tokens.
4. Only the host may lock or unlock a token.
5. Tokens that are locked and not a player token cannot be interacted with by players.