Keep off-grid position of TCOs on group moving

[PhysSong]

This is a simpler alternative of #4043. Instead of calculating relative offsets from snap points, it just quantizes the change amount of offsets.
Fixes #2300, closes #4043.

[zonkmachine]

This works fine. What about moving a single track in edit mode? Should it snap to grid or keep the offset?

[PhysSong]

What about moving a single track in edit mode? Should it snap to grid or keep the offset?

I think it should snap to grid unless modifier(s) are specified.

[zonkmachine]

I think it should snap to grid unless modifier(s) are specified.

I would probably expect not snapping effect without a modifier as that is what I get with moving a group.

Maybe you can have two snap points. Grid and offset that you step through?
Example. Offset TCO at 1.2 dragging right: 1.2 -> 2.0 -> 2.2 -> 3.0 -> 3.2 etc.
No modifier key needed.

[zonkmachine]

I think this should be merged. It's already a huge improvement. If the action needs fine tuning later then we can do it in master. As for moving single objects one can simply select it as you do with multiple objects move and it won't snap when moved. I still think moving a single object shouldn't snap however but I'm happy with this way of operation too. Also, if we merge we'll get feedback.

[zonkmachine]

Merge?

[PhysSong]

It's already a huge improvement. If the action needs fine tuning later then we can do it in master.

Merge?

We can easily revert this if there are some issues with it. If there are no objections, I'll merge this.

[PhysSong]

Merging. I hope #4357 or something will finally supersede this one.