cleanup context variables on object/context destruction

[umlaeute]

When an object that holds context-specific data (using gem::ContextData<>) is destroyed, all context-data should be destroyed in the correct context.
This involves switching to each context and freeing any resources.
e.g.

     static void GemContext::forEachContext(GemBase&obj) {
        oldctx=getCurrentContext();
        for ctx in contexts
        do
            ctx.makeCurrent();
            obj.removedContext();
        done
        oldctx.makeCurrent();
    }

Similiary, when an openGL-context is destroyed (e.g. by closing a window), each object holding context-specific data should be called to free the resources of this context:

    myContext::destroy() {
          makeCurrent();
          for obj in GemBase::objects
          do
                obj.removedContext();
          done
    }

[umlaeute]

afaict, this has been implemented with d7f5cfc..953c70f

(note that the above-mentioned GemContext::forEachContext is slightly different, as it does not restore the originally active context!)