Pathgrid edit

Not quite ready for merge yet, but could do with some reviews, feedback and maybe some play testing please.  Remaining issues include:

* saving to the document (and updating other scene subviews via signals)
* adding/deleting edges
* selection states for editing (currently the cursor needs to be above the pathgrid point for the delete key to work)
* indicies (stored in the edge structs) are not recalculated when a pathgrid point is deleted.  This may require some thinking.
* placement of the pathgrid points (whether adding or dragging) is sub-optimal