Add a stop button.

Turtle animations can now be stopped.  When running a program
that supports $.turtle.interrupt, the IDE now polls (every 0.1 sec)
$.turtle.interrupt('test') and shows a stop button when true.

Clicking on the stop button injects an interrupt() call into the
running program, stopping animation.
Merge pull request #39 from PencilCode/stopbutton

Add a stop button.