Improved speed of renderman shader menu.

The 3delight installer was adding some dubious paths, and making duplicate paths too - see comments.
Stopped GraphGadget representing connections between two plugs on the same node.

Also changed a bunch of smart pointers to raw pointers in line with our "only use smart pointers when you need to manage ownership" rule.
Fixed bug whereby Plugs were accepting inputs from themselves.

Also removed todo which was done already.
Fixed LabelPlugValueWidget bugs.

It wasn't updating the label greyed-out state correctly on construction, only on a change to the plug. It was also greying out the label when a plug had a input connection - now it only greys the label out if it has been explicitly set to read-only.
Work in progress. Just committing so I can work on it remotely.