Add save contents before system sleep feature

It's on our list for RAM File System, but not for this release - we're currently focused on the macOS and Linux versions, so it would come at a later stage. There's a technical side to it too: Windows gives a driver very little notice before it suspends, and no way to ask it to wait, so a volume of any decent size can't reliably be written in that window. We'd rather do it properly than releasing something that only works for small volumes.

In the meantime, RAM FS can already cover the case you're most likely concerned about. Each volume can save its contents automatically:

• at a periodic interval of your choosing
• before the volume is dismounted
• at system shutdown

The periodic save is the one worth enabling - with an interval of a few minutes you always have a recent copy on disk. Note that sleep and hibernation don't lose anything by themselves; the volume comes back intact on resume, so this only needs to cover an unexpected loss of power.