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