Improve dragging of nodes
Improve dragging of nodes

Dragging a node at its border doesn't make the center snap to the mouse
position anymore.
VisualDataWeb/WebVOWL,MissLoveWu/webvowl,VisualDataWeb/WebVOWL,leshek-pawlak/WebVOWL,MissLoveWu/webvowl,leshek-pawlak/WebVOWL