Initial implementation of backend side for spectating

[Bios-Marcel]

Another attempt at implementing spectating as request in #201

TODO:

• Spectators should not be visible in the player list
• Switching to / from spectating should only be possible after a turn has ended
• Spectators shouldn't be part of the game loop

  This needs to be tested well. Should potentially work already.
  Maybe integration tests would be useful here?

• There needs to be a button to request a switch to / from spectating

  For testing purposes, I just call the toggleSpectating function for now.
  This could be a button in the top-bar that switches between an eye and a controller or something like that.

• Display "Blah blah is now participating / spectating" in the chat

To consider:

• Should spectating players be kickable?

  If not, this could help to dodge an almost full kick vote. On the other hand, it might flood the kick list.

There was already an attempt at implementing this in feature/, but it is quite outdated and I changed the implementation and plans a bit anyway.

[Bios-Marcel]

The backend side of things is solid as is.

The only thing potentially annoying is the abuse of the update-players event. But maybe this is something I should generally update at some point? Cause this is used for multiple things at this point.

The frontend is a little garbage. It works, but it isn't really clear to the player what the current state is. This should instead be a dialog that displays the state and allows you to revoke your active requests.

I'll merge this for now, since I am not sure when I'll work on it again and I want to avoid merge conflicts.