show guide lines/boxes for nodes

[choey]

Hi,

This PR adds a quality-of-life improvement for keeping nodes neatly aligned in a workflow.

Problem

Consider the current behavior when organizing a node (snap to grid turned on):

demo1.mp4

Except for the faint preview box, indicating where the node will snap to, it's difficult to align a node with others (especially if they are far apart in the workflow).

Solution

1. Display orthogonal guide lines that extend across the canvas. This helps align a node with other nodes.
2. Customize the preview box for improved visibility.

demo2.mp4