Feature: The view can be moved by dragging the canvas

[wenmin92]

At present, for large graph, we can only use the scroll bar to view the complete graph, but it is not very convenient. It would be better to move the canvas directly.

[broofa]

Yup, agreed. This feature needs to not interfere with other click-actions in the graph, however, which may complicate the implementation somewhat. E.g. clicking on a node to view it in the inspector.

[wenmin92]

1. only click & hold white-space to drag
2. with modifier key, like shift or ctrl
3. or right click & hold to drag