xdg-shell: bump experimental protocol version

xdg-shell: bump experimental protocol version

The experimental versioning has not been updated when it was supposed
to. Let's try to be better at it now, as xdg-shell is close to have its
first stable version.

Bump the version now to bring the world into the same exact version.

There may be some protocol changes still coming, but we try to land them
before 1.6 gets out. Those changes will bump the experimental version
again as needed.

When 1.6.0 is released, the experimental version will no longer be
bumped, and no incompatible protocol changes will be made. Xdg-shell.xml
file will move to Wayland in 1.7.0, drop the experimental versioning,
and become stable.

Cc: Jasper St. Pierre <jstpierre@mecheye.net>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>