#117 Handling of Fragments, cut, undo etc

[Daire Kivlehan]

On cut, undo, move, sort etc of Fragments, their file
information is often lost.
This is due to the fact that the fragment is deleted
(unlinked) and then "created" or linked again at a
different place.
What is needed is a cache of old Fragments in the
DocumentImpl class that remembers what fragment
was associated with a root instance of a portion of the
objectmodel when a fragment is unlinked, so that this
Fragment will be used rather than a fresh Instance when
the root instance is linked again at a different place.

[kbe]

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This seems rather hacky. :-)

The clean solution would be to add a SingleLink class to the
objectmodel, to which this information could be associated
even when this single link is not part of a Link.

Even better: Rename the actual "Link" to "Slot" and create a
new "Link" class ...