Added a GafferScene::Set node.

This will be used to allow users to manage sets of named locations as part of their graph flow.
Typedefed PathMatcherData into GafferScene namespace.

It was always meant to be defined there, but wasn't because of problems using the Cortex macros in another namespace. The new typedef matches what we do for GafferImage::FormatData, and means we can tidy up the code referring to PathMatcherData.
wip