Only call the destructor of an includelet when the last of a set of clones
is deleted.
(my commit to layout-manager is incorrect, I simply added an boolean param
to the element delete proc that determines if the destructor is called. The
notion of cloning and the deletion of clones comes from the includelets
admin code in layout-managed-subsite, and it is this code that determines
when to call an includelet's destructor.)
