Hold space to pan graph by dragging from anywhere

[egraether]

like in Photoshop

Currently you can only pan the whole graph when clicking and dragging into the background area.

With this change, holding down space should allow to click anywhere to drag the whole graph, even when the mouse is currently over a node or edge.

Implementation suggestion:

• QtGraphNode and QtGraphEdge need to ignore the mouse drag event when space is held down
• QtGraphicsView needs to handle the drag when space is held down.