Fixed bug when right clicking a non-DependencyNode node in the node graph.
Added optional Menu titles.
Added title to PlugValueWidget popup menus.

The title disambiguates the context in which the menu items below will be executed.

Fixes #265.
Fixed menu title so it doesn't interfere with menu keyboard navigation.
Merge pull request #373 from johnhaddon/plugMenuTitle

Plug menu title
Removed right-click layout context menu.

It can now only be accessed via the layout button.
Added Box::unpromotePlug() method.
Added "Unpromote from Box" item to plug popup menu.

This also removes the "Remove input" item if it was there.

Fixes #218.
Merge pull request #376 from johnhaddon/boxUnpromotion

Box unpromotion
Image stats node in ImageView now uses the preprocessed input plug if the raw input is not an ImagePlug
Improved speed of renderman shader menu.

The 3delight installer was adding some dubious paths, and making duplicate paths too - see comments.