Constrain user movement axis with Shift

[jnslmk]

To put elements on screen in relation more easily, I propose the feature to constrain the rulter movement with the Shift key.
Dragging the ruler with the mouse while holding Shift would allow the ruler to move either up and down or left and right.

Powerpoint and other Microsoft products implements this feature with Shift.
Inkscape uses the Ctrl key instead.
I believe there are more programs offering a similar feature.

[andrijac]

Good idea

[alexis-m]

I will work on this after #36.