MAContainer not working with pragmas - regression by changes from GToolkit

[JanBliznicenko]

Since https://github.com/magritte-metamodel/magritte/commit/9842df75c0f5e728d76892fa0f3b55bdd1f6b840, descriptions using MA(Priority)Container no longer work, because not all description classes implement definingContext: sent by MAPragmaBuilder>>#setDefiningContextOf:to:in: sent by MAPragmaBuilder>>#buildDescriptions:

I believe definingContext: should be implemented by MADescription (or defined as subclass responsibility and implemented in MAContainer as well).