Possible extensions of the (SKOS) concept schemes defined in POE #160

@aisaac

1. I'd like to make a suggestion regarding possible extensions of the (SKOS) concept schemes defined in POE, such as the Actions one.
   The POE model foresees that there can be new actions, which from the perspective of POE are not supported, and thus captured as ‘undefined’. This is apparently being debated, as reflected in Undefined Actions #105. But can’t a simpler, more flexible approach be taken, which would avoid the awkwardness of this ‘undefined’? I’m thinking of the Web Annotation mechanism to represent annotation motivations: https://www.w3.org/TR/annotation-vocab/#extending-motivations .
   The rule there is that WA introduces a set of basic Motivations (like POE introduced a set of basic Actions) and WA recommends to extend this set by defining new Motivations as specializations of existing ones, using skos:broader.
   The actions already present in POE seem to cover a wide ground already, it seems that such approach of “extension-by-specialization” would be possible.