Add the cwaypoint command

[xpple]

Closes #510. Waypoint labels are displayed at the top of the screen in the direction of their location. If two waypoint labels would overlap, they are stacked above each other. Every tick I am creating a list of the $x$-offsets of the waypoint labels calculated against the waypoint location, and player's position and orientation. You could also precompute a data structure that lets you construct a sorted list from the player's position and orientation, but complexity-wise both are $\Theta(n\log(n))$ work, where $n$ is the number of waypoints.

[xpple]

The only thing that remains is actually rendering the waypoints. I'll leave that to someone else as I don't have any experience with rendering stuff like this.