randr: stop immutable output properties from being deleted (v2)

[Luc Verhaegen]

This version of the server side patch now no longer mangles the output
properties list directly. This incurs a slight bit of overhead, in what is
an extremely low frequency path anyway. It also fixes the error thrown in
case the property does not exist for the listed output, to match
QueryOutputProperty.

The new version of the randrproto.txt patch now fixes the two issues daniels
spotted and now fully documents all the errors that RRDeleteOutputProperty
might throw.

Luc Verhaegen.