Tap with two fingers to undo

[superenginegit]

Is your feature request related to a problem? Please describe.
I have become accustomed to being able to undo the last writing I did by tapping the touchscreen with two fingers, and this feature is not available in Xournal++.

Describe the solution you'd like
I would like to implement the two finger tap undo and add a setting to toggle it on if wanted, as I can see it being annoying if one does not expect it to be there.

Describe alternatives you've considered
It can be done by binding undo to a stylus button, but I typically like to use those for other tools, so it is not really a good option for me.

Additional context
I'm planning on implementing this feature and then creating a PR for it when it is ready.

[tmoerschell]

Something that would go very logically with this would be a redo shortcut, for example with a three-finger tap. Would it be worth it adding it into the same PR?