month-view: rework allocation code

GcalEventWidget added API to automatically change
the widget rendering according to the initial and
final dates. Month view, however, was still handling
it manually (and wrongly).

Fix that by reworking the allocation code, documenting
it and using the apropriate API from GcalEventWidget.

src/gcal-subscriber-view.c

@@ -239,7 +239,7 @@ gcal_subscriber_view_remove (GtkContainer *container,
       master_widget = (GtkWidget*) l->data;
       if (widget == master_widget)
         {
-          if (gcal_subscriber_view_is_child_multicell (GCAL_SUBSCRIBER_VIEW (container), GCAL_EVENT_WIDGET (widget)))
+          if (g_list_find (priv->multi_cell_children, widget) != NULL)
             {
               priv->multi_cell_children = g_list_remove (priv->multi_cell_children, widget);