add undo and redo commands

Closes #215 (closed)

Adds commands to undo/redo changes to the displayed signals. Only commands that modify displayed signals have an effect on the undo/redo operation, e.g., toggling the menu bar is not considered.

This works by cloning the data structures for the displayed items and keeping them on two stacks (one for undo and one for redo).

I'm up for discussion to which operations should be considered.