cell snapping and x2 cell width causes a lot of issues

[lgarczyn]

snapping:

* snapping is applied differently when moving multiple cells, only applying on the cell actually dragged
* this causes a gradual offset between the dragged cell and the "follower" cells

x2 width:

* dragging a single cell will cause it to suddenly move to place the center of the cell on the cursor
* new conversation have the Start cell halfway over the left edge

[Tony Li]

Snapping used to move the other nodes relative to the dragged node, which kept them in the same positions relative to the dragged node. Some users had requested the current behavior. But if no one minds, I prefer keeping them relative to the dragged node, so I'll change that back.

I'll also look into the x2 width issue.