AI Tracking bugfixes 

Fixes #4201 by adding a cancel button to the track mob list.
Fixes a runtime that pops up if you double click the "Track Mob" or "Show Camera List" buttons on the AI hud.
Problem is that double clicking runs a proc that builds the datum of lists of trackable mobs twice, and then both procs sleep because of input(). When the first track completes it nulls the tracking datum, which causes problems for the second (or more) tracks which expect the tracking datum to not be null.
Solution: Keep the datum around and simply rebuild the lists as-needed instead of creating and deleting the tracking datum pointlessly.