Add Koopa Shells

[Hugman76]

This adds Green and Red Koopa Shells to Mubble.
Koopa Shells are projectiles that slide on the ground and bounce off walls.

Auto-aim

Red Koopa Shells are capable of targetting enemies themselves. Be wary that they may target any living entity that is not on the same team as the sender!

Draft?

This PR is a draft because it is missing key features:

• Koopa Shells may break at some point
  • Green Koopa Shells should break when they hit a wall a set amount of times
  • Red Koopa Shells should break the first time they hit a wall
• The collision algorithm is kinda ugly
• Code is missing documentation
• Needs more configuration (target speed, max bounces, etc..)
• Red Shells may skip the block bouncing algortithm altogether?
• Players should be able to bounce on Koopa Shells to stop them and shoot them with the foot