Added per-app registration of editors.
* Added a python binding for GafferImage::Filter::defaultFilter()
* Added a method to GafferImage::Sampler to set and get the sample window.
* Added a constructor for the GafferImage::Sampler that does not require a filter (and subsequently uses the default filter).
* Added python bindings for the new constructor.
* Added test cases for all of the above.
Tweaked layout menu order.
Merge pull request #360 from johnhaddon/perAppEditors

Added per-app registration of editors.