The Compress and Extract service menus shown in Dolphin's context menu are now
provided by two new KFileItemAction plugins. This allows us to do a lot of
useful checks before actually showing the actions in the context menu.
This also allows us to stop hardcoding the supported mimetypes for the Extract
menu. This is actually needed in order to support the Extract menu when more
than one archive is selected. In this case Dolphin sends to the plugin the
generic 'application/octet-stream' mimetype, so we have to manually check
anyway the mimetype of any given URL.