apply the observer pattern for the gui to get notifications from AntView

apply the observer pattern for the gui to get notifications from AntView

the update method does the following:
it removes the applet from the list of observers in the Environment
(which updates the graph etc.)
then it sets antView to null since it got disposed; we check somewhere
else for it being null;
if the Environment thread is not null and hasn't terminated yet we send
it an interrupt so it can return gracefully.
when this is done we set antThread to null so we can check for it later
finally we need to reset the button text
and after everything is done we run the gc since it might take a view
seconds before a user wants to start a new computation