Moved GafferScene Options bindings to separate file.

We need to continue to thin down GafferSceneModule.cpp because it has massive compile times.
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.
Fixed InteractiveRenderManRenderTest for slower machines.
Replaced forwardDeclarations with a private set for lights.

The "gaffer:forwardDeclarations" global entry was our slightly ad-hoc way of tracking the location of lights in the scene. It is now replaced with a private set called "__lights", and all the old code for keeping forward declarations accurate when the hierarchy changes has been replaced with more general purpose code for keeping sets in sync with the hierarchy. This both makes sets more useable and pleasingly tidies up one of the more untidy corners of Gaffer.
Improved GafferScene::Set documentation.

It implied that sets only held locations which referenced objects, whereas actually they are happy to reference any location at all.
Fixed PathMatcherData references for GCC 4.1.2.
Optimised PathMatcher.

Added new methods for adding and removing paths as vector<InternedString> - this avoids all the overhead of tokenization for cases where the path already exists in tokenized form.

Changed internal representation to use InternedString rather than std::string. This is much more compact than the old representation, since most strings we put into a PathMatcher will already have an InternedString representation somewhere (from the SceneNodes or from IndexedIO).

PathMatcherTest.testLookupScaling() shows a 67% reduction in time to build the PathMatcher, a 78% reduction in time to perform the lookups, and a 70% reduction in memory usage.
Interleaved hash/compute methods in SceneReader.cpp.

It's much easier to verify that the implementations correspond appropriately when they're adjacent in the source code. We should do things this way as a matter of course in all the code.
Implemented loading of tags as sets in SceneReader.
Adjusted SceneProceduralTest for new SceneReader behaviour.

For the test, we need a node which will throw during procedural expansion. The SceneReader no longer fits that bill now that it implements computeGlobals(), as it will now throw during construction of the SceneProcedural. Using the Text node with an invalid font gives us an exception at the right time.
Improved SceneReader UI.

- Moved to a separate file. We should do this for all UIs as we go through and add metadata for each node type.
- Added Metadata.
- Added right click menu for toggling tags on and off in the tags and sets plugs.
Merge pull request #831 from johnhaddon/sets

Sets groundwork