sch: relative time spec for events

Reference calendar time of other schedules, user-created named events
or the sun{rise,set} (when enabled)

For example
- '15m before sunset'
- '30m after cal#0'
- '1h15m after "foobar"'

Empty time spec is allowed, defaults to '1m'
'before sunrise' is the same as '1m before sunrise'

Internals are reworked to handle a more generalized 'Event' type, based
on the 'time point' and 'event' base classes fron sunrise and sunset

Sunrise and sunset should also track 'last' event as well as 'next
(not currently displayed anywhere, though)