Implement PNS dragging functionality for differential pairs (lp:#1531031)

Original report created by Artsiom Shchatsko (cioma)

It would be great to have "Drag Track Keep Slope" (hotkey "D")
functionality implemented for differential pairs.

Currently when I press "D" in "Route Differential Pair" tool and try to
drag a 45 deg track of a differential pair the space between tracks of the
diffpair is not preserved.

Original tags: pcbnew