Introduce GcalEventList

This is a GListModel implementation that is specific to
GcalEvents. Events are stored in a GSequence in order of
date.

Add a couple of tests exercising additions and removals.

WIP: add more tests
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/742>

src/core/meson.build

@@ -6,6 +6,7 @@ sources += files(
   'gcal-clock.c',
   'gcal-context.c',
   'gcal-event.c',
+  'gcal-event-list.c',
   'gcal-global.c',
   'gcal-log.c',
   'gcal-manager.c',