Implement mousewheel button dragging for graph panning

[save196]

Describe what problem your feature request solves: The current graph panning method requires a combination of keyboard and mouse inputs, and mousewheel button is redundant with left mouse button.

Describe the solution you'd like: Users should be able to press down the mousewheel button and move the mouse to pan around the graph.

[jgadsden]

Thanks for the enhancement request @save196 , are you able to provide an implementation? Otherwise I am not sure when this can be worked on so I have put it down as version 3.x, which is scheduled for early next year