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