Add an option to show apparent solar time

[wbreeze]

The clock currently shows system time. Add an option to show apparent solar time. This is the time that puts the sun on the local meridian at noon. It bases time on the hour angle of the sun.

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]

Of course the sun glyph already indicates this time. With this setting, the hour hand will always point at the sun on the dial.

[wbreeze]

See definition of apparent solar time at https://en.m.wikipedia.org/wiki/Solar_time.