Implement custom data provider picking and selection highlighting.

[ppt-adsk]

Remove hard-coded picking restrictions and allow for plugins to provide picking and selection highlighting support. Demonstrated this in the Flow Viewport API data producer example plugin.

The main points in the branch are the following:

• Introduction of a path mapping registry. This takes over responsibilities from the path interface scene index: placing path mapping in a scene index has proven to be inflexible and constraining. The registry allows its clients to retrieve a path mapper based on an application Ufe::Path. The path mapper converts an application Ufe::Path into one or more (in the case of instancing) Hydra scene index prim paths, which is used to set the Hydra scene index selection, therefore providing selection highlighting. Having a global registry provide this service is much simpler. I'm unsure about the terminology: in this branch I'm calling it path mapping, because I feel that is more precise than path interface, but I'm open to suggestions. Currently my PathMapper base class simply derives from PathInterface.
• Introduction of a pick handler and associated registry. The pick handler class hierarchy was previously an implementation detail in the renderOverride.cpp implementation file, and was therefore inextensible. It has been factored out. Also, we previously used a simplistic, hard-coded scheme in renderOverride.cpp to distinguish between Maya data and USD data only, at pick time. The pick handler registry allows a plugin to provide pick handling, to convert a Hydra scene index pick result into an application selection.
• Use the path mapping registry and the pick handler registry to add selection highlighting and picking to the Flow Viewport toolkit data producer example plugin, a locator that generates cubes in the Hydra scene index scene. Augmented the locator plugin to provide hiding and translation of individual cubes. While the code isn't necessarily as performant as it could be for production use, it demonstrates the interfaces and capabilities.