Added “Stop Search” button to Find dialog

Previously wasn't possible to stop a search in progress (even
when closing the Find window).
Some further tweaks related to stopping search

We now show “Stopped.” as status text when search is prematurely terminated.
Closing window also cause search to stop.
The stop button has ⌘. as key equivalent (the standard “cancel” key).
Don’t let the stop action go via the search action dispatcher.
Force cache invalidation when deleting bundle items

This is to workaround missing fs-events. Ideally the bundle item itself would do the cache invalidation, since it’s complicated to deduce what paths need rescanning after a delete.