Re #6806. Update start time option to "N minutes ago".

This replaces having to give an absolute time in UTC (though that's still
the format used by the underlying algorithm).
This is more user-friendly and compact, though I'd expect this to be an
extremely rarely-used option in any case.
Re #6806. Tweak size to avoid widget overlap on Mac.
Capitalization correction to avoid accidental new category.

NOT Re #6806 - I just happened to spot it when I was working on that ticket.
Merge remote-tracking branch 'origin/feature/6806_startlivedata_last_n_minutes'