In GimpContainerTreeView, work around a GTK bug which can cause the
window containing the tree view to stop processing updates,
resulting in an apprarent freeze, until update-processing is
explicitly requested.  This can be triggered by adding layers to a
layer group until the layers-dialog tree view is almost full, but
still doesn't have a scrollbar, and then duplicating the group.

This doesn't seem to affect GTK3, so not applying to master.