Added public Filter methods for specifying input scene via Context.

Prior to this, it wasn't possible to query a filter from Python any more. The new methods also mean that FilteredSceneProcessor doesn't need to know anything about how filters are implemented (which is good, because it's a horrible dirty secret), and we hide some more implementation by making the name of the context entry for the input scene private.
Added SceneAlgo.h with methods for querying all objects matching a filter.

The methods are implemented using a threaded traversal of the scene for performance.
Added NodeGraph menu item for selecting objects affected by a node.

This allows a more general form of what is requested in #375, but we should probably also put something more directly in the Filter node UI at some point, to fully close that issue.
Merge pull request #863 from johnhaddon/selectFiltered

Select objects affected by a node