Simpler batch undo

[allejok96]

A temporary solution for undoing multiple things at a time.

I know the journal system will/needs to be reworked, so I tried to make a far less aggressive change than #6410. When we move to a system like #7895, this won't be needed anymore. But until then we can make life a bit easier.

The batch action works by assigning every CheckPoint a number. If consecutive checkpoints have the same number when undoing, it undoes all of them. This means creating a batch action is very cheap and very easy. Just add a single line before you start.

I haven't implemented this anywhere but in ClipView for demonstration purposes. If this gets approved, I can go on to implement it in all places from #6410.

[allejok96]

I think this will still be compatible with #7821... The only problem is the undo history will display each individual checkpoint even though they will all be undone at once.

[allejok96]

Closing since there has been no interest in fixing undo temporarily until we get commands working