shell-provider: use GcalEvent instead of event data

Following the sequence of porting commits, make the Shell's
search provider class handle events using GcalEvent classes
instead of the old GcalEventData structure.

src/gcal-application.c

@@ -563,8 +563,8 @@ gcal_application_set_uuid (GcalApplication *application,
 
 void
 gcal_application_set_initial_date (GcalApplication *application,
-                                   const icaltimetype *date)
+                                   GDateTime       *date)
 {
   g_free (application->initial_date);
-  application->initial_date = gcal_dup_icaltime (date);
+  application->initial_date = datetime_to_icaltime (date);
 }