manager: fix reference management

GcalManager:remove_event() is an asynchronous function
that, by the time the callback is executed, unreferences
the event that is being removed. This implies that, when
the function was called, a reference was taken -- we don't
want the event to be finalized during the operation, after
all.

The problem is that the reference was never taken.

Fix that by properly owning a reference to the event when
removing it, and dropping it when the removal is finished.

Issue: #268

src/gcal-manager.c

@@ -1715,7 +1715,7 @@ gcal_manager_remove_event (GcalManager           *self,
                               (ECalObjModType) mod,
                               self->async_ops,
                               on_event_removed,
-                              event);
+                              g_object_ref (event));
 
   g_free (rid);