Slowness, no undoability and exception on cancellation for "fix untagged unconnected nodes"

[bilbo]

Load attached file (manufactured test case) and run validator. You will get about 4500 untagged and unconnected nodes. Press "fix" to start fixing them. You'll see progressbar (at my machine going at about 100 nodes deleted per second)

I think 100 nodes/sec are quite slow (45 secs to delete them all), because if I just select all the 4500 nodes and press "d" they disappear in just about 2 seconds.

But what is worse: If I press cancel in middle of deleting the nodes, then some nodes will be deleted, but no "undo" item will be added to the command stack (not good, there should be undoable item with part of the nodes that were deleted) and pressing "Fix" again will cause exception in the plugin (because it is trying to delete already deleted nodes)

If I let the validator finish, it will create about 4500 sequences, each one deleting single node. I think it would be better to create one sequence and put all deleted nodes in it. This way it is impossible to simply undo the fixing, as you'll have to press "undo" 4500 times...

[bilbo]

Test case