Add an option to show Mean Solar Time

[wbreeze]

The clock currently shows system time. Add an option to show mean solar time. This is the time based on current longitude.

This requires location. Fall back on system time when location is unavailable. Make the option selectable only if location is available.

Depends on #24

[wbreeze]

See definition of mean solar time at https://en.m.wikipedia.org/wiki/Solar_time. The spec’ for this issue is essentially correct. The time UT1 is essentially the same as UTC. See https://en.m.wikipedia.org/wiki/Universal_Time