core.controller, gui.edit.schemaeditor: remove the option to create x-advene-zone content

x-advene-zone is now deprecated, the standard SVG module should be used. Support code is preserved for backwards compatibility.

git-svn-id: svn+ssh://svn.gna.org/svn/advene/trunk@7624 3fb6f576-06d4-0310-ad11-a4b695aa9f05