Began implementing the second application task which is filtering the
root index according to the user-provided filter strings. The task takes
place inside a SwingWorker thread FilterIndexTask in ordert to separate
it from the EDT. Refactored the abort task code; upon completion each
task would explicitely update the UI and member objects to reflect a
change in the program's status, and now those occurences are calls to a
method abortTask() in FileFinderApplication.