Lock axis when panning

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.

Start by locating the canvas panning entry points for MMB+drag, Space+drag, and the tilted viewport gestures using Alt. Trace how Shift-modified movement is interpreted and how viewport axes are represented. Done means Shift constrains panning to the viewport's X or Y axis, including when the document is tilted.

Written by the indexing model from the issue text.