Add setting for key shortcut only for moving viewport, without draging nodes

[ToP29]

I always try to move viewport by draging while holding middle mouse, but when I am accidentally over Node, it drags the node instead. I didnt find options to use only left mouse for dragging nodes and middle mouse only for moving view

[McBroColi]

This is a key component of a graph editor and would be great to be implemented. I would rather not try to implement my own click handler for the graph over the graph and set up another layer of click handlers to avoid the issue. It would be really great to see customization of all these mouse actions supported on the graph and the contents of the graph.