libgimp: fix GimpPlugInClass methods introspection.
Commit 005143a4 completely broke all the introspected plug-ins, so I am reverting only the relevant changes. If some things need to be improved in this part of the docs, let's do it without breaking major stuff. In particular: abstract methods must be properly documented (with parameters, return values and annotations) like any function, and must be named Class::method, e.g. GimpPlugInClass::query_procedures. Other we break GObject Introspection, i.e. we break all plug-ins using these.
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