issues with undo history

[jonorthwash]

I'm closing #39 and transferring the remaining issues with undo/redo here. Feel free to add more.

• [x] Add keyboard shortcut support (ctrl+z and ctrl+y/ctrl+shift+z). Overlaps with #135.
• [ ] Support for remembering the history when things are changed in the textbox.
• [ ] the following issue from Kadri:

  I just managed to mess up a sentence in a file I used for exploring the program and the 'undo' button seems not to work. It works ok for text, but not for the visualized tree.

[adrianeboyd]

• If you delete multiple arcs at once, undo redoes them all with the label of one (it seems to choose the label of the rightmost one)

• In some cases, if you create an arc between two words that already have heads, undo only undoes the insertion of the new arc, not the deletion of the existing arc (multiple undo steps don't help, either). I'm not sure exactly what's going on, but I can say, if you open UD German dev and add an arc from ist to ein, this happens. It seems to happen when both words have the same head, but that's the not the only issue.

[jonorthwash]

For the second one, I think it's because undo history only tracks explicit actions (like the linking of a node), not implicit ones (like removing an existing link when you specify a new one). It needs to track both.

[adrianeboyd]

Sometimes it works, though, and I can't figure out the exact difference between cases where it works and cases where it doesn't.