in-game help overlay

[mabre]

• triggered by F1
• list of in-game shortcuts
  note that clients must be able to show help, too, but cannot pause the game

[Jessypet]

5bed26a Everything working in local game and also when the host is pausing the game. When a client presses F1, it says Help instead of Pause and the game is not paused, but the overlay is shown in the hosts window, not in the clients window, don't know where to fix that.

[mabre]

Showing the help is a special cause which must be handled in the event filter, since the server does not need to know when a client wants to see the help.