Export refactor

Cleanup of writer's handled()/registered()/create() methods
added file format selector combobox in dialog on export
Adds the right suffix when selecting an exporter

Also, fixed a memory leak, and removed the filter list with a hack
Added supportedExtensions() methods for writers missing one
Change back a patch that broke the priority list, and display the exporters' list according to priority order
Fix merge pb after rebase, remove 2D exporters
Some cleanup in itkDataWriterPlugin
Removed the Philips REC writer, which doesn't work with 3D images
Merge pull request #113 from fvichot/writers-refactor

Export refactor