month-view: use the event's dates instead of widget's ones

Conceptually, we need to compare the event's dates to make
sure we're allocating the widgets correctly, as widget's
dates may not represent the event's ones.

src/gcal-month-view.c

@@ -1105,7 +1105,7 @@ gcal_month_view_size_allocate (GtkWidget     *widget,
        * month. Otherwise, the first cell is the 1st day of the month.
        */
       j = 1;
-      date = gcal_event_widget_get_date_start (l->data);
+      date = gcal_event_get_date_start (event);
 
       if (g_date_time_get_month (date) == priv->date->month)
         j = g_date_time_get_day_of_month (date);
@@ -1123,7 +1123,7 @@ gcal_month_view_size_allocate (GtkWidget     *widget,
        * if the event is all day or not.
        */
       j = icaltime_days_in_month (priv->date->month, priv->date->year);
-      date = gcal_event_widget_get_date_end (GCAL_EVENT_WIDGET (child_widget));
+      date = gcal_event_get_date_end (event);
 
       if (g_date_time_get_month (date) == priv->date->month)
         {