Add a stop button to the mask tracker

[jaydude-wb]

It's unclear how to stop it. Why not add a simple stop button?

[Rick Gerard]

All you have to do is click any one of the Analyze buttons in the Tracker panel. Tracking will stop. 

[jaydude-wb]

It's unclear UI, and contra intuitive. 

Why not add a stop button, like in every other playback control...