Added "Delete screenshot" option && solved bug after deleting

I think I've managed to solve it correctly
As the ScreenshotMonitor doesn't receive an event which can indicate if there has been an addition or a removal, I implemented a method "isDeletion(), which compares the blacklist length with the query results length.

When I tried to access the new query results after a deletion, it crashed, so I don't remove the deleted screenshot from the blacklist, but as its name is created based on the current time, it won't be added again