Simplified OpDialogue error display.

We were putting the entire error message inside a label, which meant that the window could become extremely large if the error was long. We now simply display a short "Failed" message and rely on the Details pane to provide the full information - this is the same thing we do for successful operation - there is a short "Completed" message and the result is displayed in the details pane.
Added a space plug to GafferScene.Transform.

This allows the user to specify whether the transform is applied in world space (the previous behaviour and the default) or object space.
Removed unnecessary forward declaration from Transform2DPlug.h

It dates back to a time when Transform2DPlug introduced a dependency on GafferImage within Gaffer, which it should never have done. That has been long since fixed, but it seems we didn't remove the forward declaration when we made the fix.
Added pivot to TransformPlug.

Also added optimised plug accessors (getting by index rather than by name).

Fixes #156.
Merge pull request #711 from johnhaddon/transformImprovements

Transform improvements
Merge pull request #710 from johnhaddon/opDialogueErrorMessages

Simplified OpDialogue error display.