Knock off minigame fixes

Adds a Timer at the beginning so that you can get ready before the minigame starts.

Various physics fixes, that caused weird behavior. E. g. it was the best tactic to press and release the movement controls rapidly, because this somehow made you stronger. This is now fixed.

Water should be properly rendered now. Although this was only visible in the editor and not in the minigame.

Fixes the edge detection used by the AI to determine who is closest to falling off. The AI will now properly prioritize the players to knock off. I don't know how this hasn't been noticed all this time, as that code was never really working to begin with.

Moved that code from the players script to the main minigame script so that it is only calculated once and not once for each player.

It should be no longer possible for AI players to stall the game infinitely. After the last human players falls off (or there is none), one AI players becomes stronger than the rest. Also the minigame should stall less in general, due to the physics improvements. Resolves #149 (closed)

Fixes some leftover refactor stuff from !267 (merged)