Introduce the new calendar monitoring API

It is, as expected, heavily inspired by the ECalDataModel code,
except it uses GDateTime, is timezone aware, and can handle all
day events correctly.

src/meson.build

@@ -93,6 +93,7 @@ calendar_incs += contrib_incs
 
 sources = files(
   'core/gcal-calendar.c',
+  'core/gcal-calendar-monitor.c',
   'core/gcal-clock.c',
   'core/gcal-context.c',
   'core/gcal-event.c',
@@ -101,6 +102,8 @@ sources = files(
   'core/gcal-night-light-monitor.c',
   'core/gcal-recurrence.c',
   'core/gcal-shell-search-provider.c',
+  'core/gcal-timeline.c',
+  'core/gcal-timeline-subscriber.c',
   'core/gcal-timer.c',
   'core/gcal-time-zone-monitor.c',
   'gui/calendar-management/gcal-calendar-management-dialog.c',