Undo system broken (pressing Ctrl-Z causes a SEGV eventually)

[yurivict]

How to reproduce:

1. Paste the first cell text from https://cadabra.science/notebooks/fierz.html into Cadabra2
2. Press execute all cells
3. Paste the second cell text from https://cadabra.science/notebooks/fierz.html into Cadabra2
4. Press execute all cells
5. Paste the third cell text from https://cadabra.science/notebooks/fierz.html into Cadabra2
6. Press execute all cells
7. Press Ctrl-Z many times. This causes a SEGV.

Terminal has this:

$ cadabra2-gtk 
Monitor scale factor = 1
No visual cell for 17051094049197271354
No visual cell for 6340952625304051051
No visual cell for 1513570937786648132
No visual cell for 18413313239140586957
Bus error

[kpeeters]

I am aware of some issues with the undo/redo system, will get to that once 2.5.x has stabilised a bit more.

[kpeeters]

Fixed with 0ac02c9920, feel free to re-open if I was too optimistic.