Framerate independence

At the moment, whenever there is a drop in frames the characters noticeably slow down. When running in debug mode, it is common (at least on my laptop) for framerates to occasionally drop down to 30-40 fps. This causes a very noticeable and irritating jank in the characters' movements.

It would be desirable if the movements and animations advanced as a function of time passed, rather than number of frames passed.

- Entity velocities should be expressed in m/s, rather than m/frame
- Translation should be updated by adding velocity times delta seconds

I'd like to give this a go, if nobody is working on this yet. Any input is welcome. :)