protocol: add support for cross-interface enum attributes

The enum attribute, for which scanner support was introduced in
1771299a, can be used to link message arguments to <enum>s. However,
some arguments refer to <enum>s in a different <interface>.

This adds scanner support for referring to an <enum> in a different
<interface> using dot notation. It also sets the attributes in this
style in the wayland XML protocol (wl_shm_pool::create_buffer::format
to wl_shm::format, and wl_surface::set_buffer_transform::transform to
wl_output::transform), and updates the documentation XSL so that this
new style is supported.

Changes since v2:
 - add object:: prefix for all enumerations in the documentation
 - fix whitespace in scanner.c
 - minor code fixup to return early and avoid casts in scanner.c

Changes since v1:
 - several implementation bugs fixed

Signed-off-by: Auke Booij <auke@tulcod.com>
Reviewed-by: Nils Christopher Brause <nilschrbrause@googlemail.com>
Reviewed-by: Bill Spitzak <spitzak@gmail.com>
[Pekka: rebased across cde251a1]
[Pekka: wrap lines and space fixes in scanner.c]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>