Add Undo/Redo support for widget.Entry

[andydotxyz]

Required for #434

[andydotxyz]

As this ticket add to the entry popup

[AndrewWPhillips]

I think this should not just be changes to the text but also changes to the selection, in which case it may even be required for read-only entry widgets.

[andydotxyz]

Undoing selection sounds a little uncommon? If I make a change then copy 3 different bits of text then undo has to be executed 4 times to undo the actual change I intended...