Fix connector dragging  & selection when view is scrolled vertically or horizontally

Fixes issue when your view has some X or Y scroll - in such case, lines for new connector and selection rectangle would be off (see cursor vs. line when dragging on screenshot below):
![image](https://cloud.githubusercontent.com/assets/5679032/3787158/02da45fc-1a04-11e4-9ef7-36ac54d79b9d.png)