Drag-and-drop is the only way to move a ruler guide today, which makes
hitting an exact pixel painful. Double-clicking the guide pill now
swaps the position label for a numeric input — Enter commits, Escape
cancels — so users can type a precise value relative to the guide's
frame (or canvas).
Closes#2311
Signed-off-by: eureka0928 <meobius123@gmail.com>