In order to be able to test the scheduling of entries over time, we need
to have control over time itself. Earlier versions of this patch
implemented a flux capacitor and a suitable framework for getting the
unit tests up to 88mph. However, that approach was found to be not
maintainable long term, and was thrown in flames into a ravine.

Instead, we factor out g_date_time_new_now_local() into a MwsClockSystem
implementation of a new MwsClock interface. As well as getting the
current time, the interface supports adding/removing ‘alarms’ for
functions to be invoked at a certain wall clock time in future. The
system implementation uses g_timeout_source_new_seconds() for this.

MwsClock includes provisions for notifying consumers of the fact that
the system time zone or system clock has changed, but these are not
currently used.

This commit, while touching an unholy number of lines of code,
introduces no functional changes.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://phabricator.endlessm.com/T20423