Add support for automatically backing up editor state to non-volatile storage

[adamkewley]

Estimate: 3-5d (bulk of the work is in establishing temporary locations, expiring files, when to persist, etc.)

E.g. if the user is editing/making a model but hasn't saved in a while, it might be advantageous to save the model to some temporary location that, when the user reboots OSC, a popup comes up saying "oi, you have unsaved changes, what do you want to do about that?" or something along those lines.

The benefit of a feature like this is that it makes unhandle-able crashes (e.g. segfaults) a little less painful. At the moment, the system can only try to automatically recover from runtime exceptions.