Implemented Path Finding, Buildings

Added rudimentary path finding based on A* as well as a Tile-based
building registry. At the same time, the building's position has been
adapted & rounded; they now fit in with the grid
For experimental purposes, the units now have a slightly smaller,
capsular collider resulting in weird spins from time to time.
Bugs:
- Path optimization only works with rays, actual units might not fit
- If a unit collides, it will spin without reason when it reached its
destination