Implement a generic proc splitLink { link node_type } for splitting a link

Implement a generic proc splitLink { link node_type } for splitting a link
in two.  The proc creates two nodes of type $node_type and replaces the
current $link with two new ones, connecting existing link endpoints to the
newly created nodes.

Implement a "move to another canvas" operation on selected nodes.

Speedup the loadCfg proc by a factor of two by replacing iterative removal
of existing nodes / links with an atomic clearing of *_list globals.  This
particularly speeds up the undo / redo operations which rely on loadCfg.

TODO: merging of cross-canvas links when moving existing nodes from one
canvas to another.  NB this means currently moving "pseudo" nodes over
cross-canvas boundaries is not yet handled properly and will yield
undefined / unexpected results.

Bug found by:
Submitted by:
Requested by:	Gordan Gledec
Reviewed by:
Approved by:
Obtained from: