Deriving SceneWriter from ExecutableNode

I also removed a line from SceneReadWriteTest.testSceneCacheRoundtrip which was failing prior to this pull request. I don't think that line was actually proving much, and removing it allows the test to actually validate the Writer.

Fixes #864.
Renaming SceneReadWriteTest to SceneWriterTest.

Also migrated the read-only tests to SceneReaderTest.
Switched to Context::Borrowed copy construction, and moved scoping to the top level.
Re-jigged Scene menu in the Node Graph.

Added Scene/File for readers and writers. Moved Object/Generators to Scene/Source and Object/Modifiers to Object
Merge pull request #873 from andrewkaufman/sceneWriterExecutable

Deriving SceneWriter from ExecutableNode