ExecutableNodes now accept Boxes as requirements inputs and outputs.
Dispatchers accept Boxes for direct dispatching.

Fixes #925.
Addressing comments
Merge pull request #968 from andrewkaufman/boxDispatch

Dispatchers accept Boxes
Added missing includes to manifest.

These are necessary to build Gaffer, so must be included in the dependency package.
Removed unused variables from image libraries.
Clang compatibility fixes.
Renamed Displays plugs and changed keys used to store displays.

- Renamed label -> name. It was strange having a UI term used in a non-UI library.
- Renamed name -> fileName. This is more self explanatory and matches user expectations.
- Used name rather than fileName to distinguish different displays in the globals data.
Added name() method to SceneInspector.Inspector class.

This makes inspectors more self contained, providing all the information needed by the DiffRow class.
Added SceneInspector.DiffColumn class.

This allows the attributes and options sections to share implementation.
Fixed dirty propagation bug in GafferScene.Displays.

Dirtiness wasn't being signalled when adding a new display, only when editing an existing one.
Added displays section to SceneInspector.

Fixes #921.
Fixed display of single empty bounding box in SceneInspector.

Previously the huge negative max and positive min where being displayed - now a simple "Empty" string is instead.