Handling of game start

[CrsiX]

The following is the game start flow:

1. There exists a lobby with an owner and some other players (maybe zero other players as well)
2. Owner clicks on a "game start" button, which makes a request to the API POST /api/v2/lobbies/{uuid}/start that forbids further actions in the lobby (e.g. joining, kicking, ...) and returns a game UUID. This also triggers the GameStarted WebSocket message, so that the other players know the game will start soon (they can display "waiting for host" message).
3. Meanwhile, the owner generates the map and the initial game state and pushes to PUT /api/v2/games/{uuid}. This triggers the UpdateGameData WebSocket message, which allows all players to see the initial map, respectively their allowed area. The game state includes the order of players, so this is no problem; the client should display "waiting for player A", "waiting for player B" and so on (this is currently the case as well). The owner might not be the first player to have his turn, though. So it must be ensured that this works by using the order of players from the NewGameScreen menu.

[CrsiX]

This behavior (without callback events) has been tested and is working now.