add element `rs` to MEI works schema for convenient linking to entities

[peterstadler]

e.g. in A020649 @solveigschreiter needs to mark up "Demophoon" (and add a @dbkey="A021411") in the history element:

<p>eigene Umarbeitung seiner Oper "Demophoon" (UA 1811 in München)</p>

[peterstadler]

There's already <ref> which can be used together with the private URI scheme "wega:" within the @target attribute. E.g.

<p>eigene Umarbeitung seiner Oper <q><ref target="wega:A021411">Demophoon</ref></q> (UA 1811 in München)</p>