wayland: Be consistent about #include-guard names

We had a mix of inconsistent names, some of which were non-conformant.
Standardize on all-uppercase-and-underscore naming convention.

https://bugs.freedesktop.org/show_bug.cgi?id=70679
configure.ac: Bump version to 1.3.90 for the master branch
protocol: validate the protocol against a dtd

The scanner is not very forgiving if the protocol doesn't match it's
expectations and crashes without much of a notice. Thus, validate the protocol
against a DTD.

Move the protocol subdir forward so we validate first before trying anything
else, and install the DTD so we can validate weston's protocols as well.
xdg_shell: Add a new shell protocol.

xdg_shell is a protocol aimed to substitute wl_shell in the long term,
but will not be part of the wayland core protocol. It starts as a
non-stable API, aimed to be used as a development place at first, and
once features are defined as required by several desktop shells, we can
finally make it stable.