Cleanup handling of temporary files

- Moves the logic of creating and cleaning up temporary files (and folders) from \OC_Helper to it's own class
- Create temp files with a prefix so we can recognize them
- Let cron cleanup any dangling temporary files

For #11401

cc @DeepDiver1975 @PVince81 @th3fallen 
Apparently I added it locally some time ago when messing around with 3rdparty stuff, it's not in 3rdparty atm