The "undo" example didn't show anything that the "undoframework"
example doesn't, and the latter is more comprehensive and properly
documented. "undoframework" also uses QGraphicsView instead of
inventing its own diagram widget.
However, the "undo" example created a nicer UI with toolbuttons,
icons, and the undo view in a dock widget, so reuse those elements
in the "undoframework" example instead.
Update the documentation quoting tags accordingly, and clean up
a bit.
Pick-to: 6.5
Change-Id: I3c91feecbd5fe3e5900838b0b51f9fe7bd190280
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>
