event-editor/event-editor-dialog: Explicitly grab focus on summary section

As described in the previous commit, this approach is more
predictable because the focus is decided in the same place.

The focus grabbing behavior is set after notifying
the `GcalEventEditorDialog:event` property as otherwise it would
not select the entire text in some occasions.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/782>

src/gui/event-editor/gcal-event-editor-dialog.c

@@ -696,9 +696,6 @@ gcal_event_editor_dialog_set_event (GcalEventEditorDialog *self,
   self->event_is_new = new_event;
   gtk_widget_set_visible (self->delete_group, !new_event && self->writable);
 
-  if (!self->writable)
-    gtk_widget_grab_focus (self->done_button);
-
   /* fill attendees list */
   self->attendees_model = gcal_event_get_attendees (self->event);
 
@@ -713,5 +710,10 @@ out:
 
   g_object_notify_by_pspec (G_OBJECT (self), properties[PROP_EVENT]);
 
+  if (self->writable)
+    gtk_widget_grab_focus (GTK_WIDGET (self->summary_section));
+  else
+    gtk_widget_grab_focus (self->done_button);
+
   GCAL_EXIT;
 }