GafferCortex

This introduces a new GafferCortex module and associated UI and tests modules. These contain the code related to ParameterisedHolders and ParameterHandlers - so all the user visible features of Cortex like Ops and Procedurals. This tidies up the main Gaffer namespace considerably, hopefully making the core concepts of nodes and plugs much more evident, and not confusing things with parameters and wotnot.

The Gaffer core still remains very much dependent on Cortex and always will, but less so on the higher level aspects such as Ops etc. Over time I expect we'll have a very much slimmed down IECore library which will contain just the basic data types that Gaffer needs, and the higher level bits will be in secondary Cortex libraries which will only be used in GafferCortex.