Fix crash on undo after importing assistant set
Before this commit, importing assistant set was not undoable, which broke internal state of the tool, which led to crash. This commit makes importing the assistant set undoable, preventing the crash.
(Taken from the bug report) 1.Create a ruler 2.Save ruler file locally（.paintingassistant） 3.Click the icon to delete the ruler (do not "delete all") 4.Import the ruler file 5.undo (Krita shouldn't crash, just undo)
Note: importing the assistant set is treated as one action, so if the user undoes it, all of them get deleted, and when they redo, all of them come back. That's expected.
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