tagFn must return true if the key event is used.
Clean up activateNode function a bit; added documentation and comments,
and tried to remove all redundant operations.
Potentially fix an annoying error, by which the new node is not made active
when pushing 'z' to interpolate nodes.
Remove unnecessary repainting in refreshAllNodes()
createInterpolatedNode ('z' key) works properly with multiple invocations.

While at it, I merged the createInterpolatedNode and createTreenodeLinkInterpolated
functions, which were almost identical and could be chained in their invocation,
which requires the chained invocation of continuations that I implemented.