gdk: Sanitize GdkContentFormats API

gdk: Sanitize GdkContentFormats API

Make sure the API reflects the idea that GdkContentFormats is a set
containing mime types. In particular, treat the object itself as a
plural - it's named content format`S' after all - and therefor use
the correct verb form.

Also make GdkContentFormats keep an array instead of a list, now that
it's immutable.