Fix bug #6923 (Brackets freezes for a while after saving when Find in Files
panel is open) - Ensure we're not accumulating many copies of the FileSystem
"change" listener.

jQuery happily adds duplicate listeners (even though, confusingly, removal
with .off() removes all duplicates at once). Since _showSearchResults() is
re-run any time you page through search results and many times when you're
making text edits near a search result, you can easily accumulate 1000s of
copies of the listener. On the next save or external change, Brackets will
run all the listeners back to back, in some cases freezing for a minute or
more.
Merge pull request #7106 from adobe/pflynn/findinfiles-hang

Fix bug #6923: Brackets freezes for a while after saving when Find in Files  panel is open