GMarkup: add G_MARKUP_IGNORE_QUALIFIED

Add a flag to GMarkupParserFlags to ignore qualified tags (along with
their contents) and attributes.

This will provide a nice way for some of our parsers (GDBus
introspection, GSettings schema, etc) to ignore additional tags that
users have added to their files, under a different namespace.

https://bugzilla.gnome.org/show_bug.cgi?id=665634