Feature request: Autosave

[jintakhan]

This was requested back in #20 but never implemented. It would be immensely helpful to have a guarantee that the GPX files are regularly being logged to an actual file rather than depend on an unreliable crash recovery. Adjustable save rate should allow the user to control the energy impact of this feature.

Or even better: when starting a new session, immediately prompt user to save to file, then once tracking starts, automatically save the session to the file at the specified rate while tracking is active.