Make semantic history and URLAction construction asynchronous. Move filesystem poking into its own thread to avoid wedging the main thread when we encounter a network filesystem.

- ARCify iTermURLActionFactory
- Make iTermURLActionFactory async
- Make users of URLActionFactory async
- ARCify some PTYTextView code. Put it in its own category since ARCifying this whole thing at once is too much
- Make open target async
- Move updateCursor into ARC and make it async
- Move handle drag of semantic history item into ARC and make async. Also fix a bug with pasting a file where it would do multiple pastes, some of which might prompt for permission
- Move quicklook to ARC
- Remove the last vestiges of URLAction from PTYTextView.m
- Do semantic history in separate threads.
- Cache whether files exist for up to 10 seconds to make semantic history faster
- Use interpolated strings in semantic history instead of a gross hack. Make openPath: async. ARCify iTermSemanticHistoryController
- Evaluate autolog filename asynchronously using interpolated strings instead of hacks. Fix how expression evaluators' lifetimes are handled to be sane.