Lock axis when panning

[Keavon]

While panning the canvas (with MMB+drag or Space+drag), add support for locking to the X or Y axis with the Shift modifier key held down.

If the viewport is tilted (with Alt+MMB drag or Alt+Space+LMB drag), it should remain aligned with the X and Y axes of the viewport not the tilted document.

[TheUnlocked]

I've tried to implement this, but what I found is that it's hard to support this in a way that interacts well with other panning shortcuts. For example, if you try to drag-pan and scroll-pan at the same time, right now both actions will add together (though cancelling the drag will also reset both). If the user can constrain the pan to an X/Y offset from the initial drag position at any time, allowing the user to scroll to pan during the same operation would be very weird and it's hard to even define how it should work.

My implementation (#3275) just has drag-panning override panning from other actions, but that is a breaking change since it affects drag-panning when you aren't locking the pan to an axis too.

[Keavon]

I will have to explore the nuances you brought up in closer detail in a couple of weeks when I'm back to focusing on managing PRs. Sorry to make you wait on progressing.