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/core/gcal-event.c

@@ -1829,3 +1829,24 @@ gcal_event_format_date (GcalEvent *self)
 
   return g_steal_pointer (&formatted_string);
 }
+
+gboolean
+gcal_event_is_within_range (GcalEvent *self,
+                            GDateTime *range_start,
+                            GDateTime *range_end)
+{
+  GDateTime *event_start;
+  GDateTime *event_end;
+
+  event_start = gcal_event_get_date_start (self);
+  event_end = gcal_event_get_date_end (self);
+
+  if (gcal_event_get_all_day (self))
+    {
+      return gcal_date_time_compare_date (range_end, event_start) < 0 &&
+             gcal_date_time_compare_date (range_start, event_end) > 0;
+    }
+
+  return g_date_time_compare (range_end, event_start) > 0 &&
+         g_date_time_compare (event_end, range_start) > 0;
+}