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 - mouse picking the edges is going to be difficult because a bullet shape and loader classes need to be built (maybe possible to get by just with Ogre without bullet?)
* selection states for editing (currently the cursor needs to be above the pathgrid point for the delete key to work)
* pathgrid point vector indicies (stored in the edge structs) are not recalculated when a pathgrid point is deleted.  This may require some thinking since both points and edges are stored in vectors (as opposed to maps)
* ~~placement of the pathgrid points (whether adding or dragging) is sub-optimal - this needs 'snap' or 'drop' capability~~
* code needs a good cleanup/refactoring & removal of debug code

To test (cell -2, 6 is a good one for testing):

* drag works like objects, via holding down the right mouse button
* to add points press 'insert' key while the scene widget has the focus - the pathgrid point is placed where the cursor is pointing
* to delete place the cursor above the point being deleted and press the 'delete' key (the scene widget needs the focus, just hovering the mouse over it won't work - try clicking the scene)