Make the file collection phase cancellable

[hsoft]

From GS

Currently, the worker thread, if in the middle of the file collection phase, won't stop until it's finished. However, some collections take a long time to complete and this makes dupeGuru unusable for a long while. Even though the progress of file collection can't be quantified, the loop should still check for job cancellation.

[hsoft]

(from [4ec173c05aff]) [#168 state:fixed] Made the file collection phase cancellable.
https://bitbucket.org/hsoft/dupeguru/changeset/4ec173c05aff/