Horde mode

Hello everyone. So after thinking about it for I while, I have finally added the first playable version of the horde mode: fight against an endless horde of units clashing against your defenses. Enemy units will spawn in batches, and these batches spawn in waves. Each wave, the number of batches and the number of units in the batch is increasing - until your defenses inevitably fail. There is no way to win, just to hold out as long as possible. On the other hand, the enemy can only "uncapture" spawn-circles and towers, but it cannot produce even more units in them or use the towers.

This new game mode is by no means finished. Lots of work still has to go into the AI, the scoring system and balancing. The AI does not use tank modes, and does not try to exploit weaknesses in the players defenses. Also, it cannot use power-ups. I presume the whole thing will get a whole lot more interesting once it does.

Right now, there's a huge difficulty spike around wave 10. If you somehow manage to get to wave 12 or 13, I'd like very much to get video proof of that.

Other changes:

• Preliminary Discord presence integration
• Fixed a bug that prevented a 4th human player from joining a game.
• Fixed a bug in the path-finding code the resulted in the path going through 0, 0