Fixed SectionedCompoundDataPlugValueWidget child widget bugs.

It wasn't implementing childPlugValueWidget(), or setReadOnly(), both of which are required for any PlugValueWidgets which have PlugValueWidget children.

Fixes #588.
Fixed Dialogue positioning and focussing on Linux.

Fixes #220. Fixes #642. I suspect this could also sort out #62, but that needs to be verified by a KDE user.
Fixed PrimitiveVariableProcessor::affects().

It was neglecting to call the base class.
Added DeleteAttributes node to GafferScene.

This derives from a new AttributeProcessor base class which simplifies the development of other nodes which solely process the values of certain named attributes. Also moved the bindings for all the attributes nodes into a separate file - the GafferSceneModule.cpp compile times are ridiculous, so we'll need to continue to split off logical groupings of nodes into separate functions like this until they're not so ridiculous.

Fixes #587.
Merge pull request #643 from johnhaddon/sectionedChildWidgets

Fixed SectionedCompoundDataPlugValueWidget child widget bugs.
Merge pull request #644 from johnhaddon/dialoguePositioning

Fixed Dialogue positioning and focussing on Linux.
Merge pull request #645 from johnhaddon/deleteAttributes

DeleteAttributes node
Fixed RenderManShader.acceptsInput() crash

RenderManShader.acceptsInput used to crash when you called acceptsInput( None ) on one of its input plugs. This was bad. Now it doesn't. This is good.
added some checks for null plugs in Box's plug promotion methods
Merge pull request #647 from davidsminor/renderManShaderAcceptsInputFix

Fixed some null plug crashes
Bumped version to 0.85.0 and updated Changes
updating default Cortex version for public build
Removed unecessary format calls.