xdg-output: make xdg_output.description mutable

The output description is a human-readable text describing the output. Unlike
the name which uniquely identifies the output, it's intended to be displayed to
the user.

It might be desirable for a compositor to update an output's description. For
instance, when only one output is plugged in, it's not necessary to dump make,
model, serial and connector to the description, something like "Dell U2717D" is
enough. However when two identical outputs are plugged in it's necessary to add
e.g. the connector type to tell them apart ("Dell U2717D on HDMI"). See [1] for
a discussion about this.

This commit bumps xdg_output's version to allow compositors to update the
property.

[1]: https://github.com/swaywm/wlroots/issues/1623



Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>