Remove --allow-unprefixed option to the scanner, and fix resulting
problems:
* theme.h and boxes.h are split into a main -header and a private
header that includes stuff that is not generally useful and
hard to introspect. Merge theme-parser.h into theme.h.
* meta_display_get_atom() and meta_window_get_window_type_atom()
are marked as (skip)
* Fix annotation: (element-type Strut) => (element-type Meta.Strut)
https://bugzilla.gnome.org/show_bug.cgi?id=632494
