[plot] Add snaping to scatter profile drag

[t20100]

Proposition from @vasole to constrain scatter profile drag to direction perpendicular to the profile line. Maybe others wants other constraints: such as move only horizontally or vertically....

Options: allow to use arrows to move the profile (then focus between plot and profile is needed), use modifier key(s) to choose constrain (complex for the user).

[vasole]

The option of using the keys is fine. Let's wait to see if more people is interested prior to implement it.