Added playback controls to spell casting

Hi everyone,

I've just pushed build 120 of the game. This contains new controls under the "cast spell" or spell runner page. This will let you play / pause / step forward / step backwards while watching your spell run.

This should speed up debugging of spells, especially those that take a while to run.

You can control playback using the mouse buttons in the corner, or use the keyboard: space / left arrow / right arrow.