Fix Go → Current Document with no project folder

We were creating an NSURL from the project folder which would throw an exception when the project folder was nil.
Copy incremental search string

Fixed #631 (again).
Improve heuristic for swipe gestures

We now try to find a scrollable view under the mouse, and if any scrolling was done, we reject the gesture as a “go back/forward” event.

Additionally we reject gestures with a duration of more than 0.2 seconds.

Maybe fixes #854.