Enable indexing contents of diagrams

[debrecenics]

• MD environment option is defined to enable disable this feature (as the engine needs to index a lot more elements than before, it could cause performance issues.)
• Reference filter is updated to include the _representation feature in the indexing phase if the option is enabled.
• ViatraQueryAdapter, MagicDrawProjectScope and BaseIndexOptions are modified accordingly.

Closes #63

[ujhelyiz]

On the code level, this change seems reasonable.

However, @debrecenics reported in private that the _representation feature does not send correct change notifications, thus the results are not updated when the model changes - which is in my opinion a blocker issue for this change, as it goes against the ideas of the project.

[tamasborbas]

On the code level, this change seems reasonable.

However, @debrecenics reported in private that the _representation feature does not send correct change notifications, thus the results are not updated when the model changes - which is, in my opinion, a blocker issue for this change, as it goes against the ideas of the project.

In the meantime: we really need the _representation in some cases and would accept that the incrementality will be limited. E.g. SAIC has a rule that checks if every block has an IBD diagram and the type of the diagram only stored in the AbstractDiagramRepresentationObject. We discussed with @debrecenics and @bergmanngabor that MD only sends the notifications after certain actions (e.g. saving or switching the focus to another diagram), but we could accept this. Probably the MD environment option shall show a warning about the consequences, and even when the option is enabled, we can put a warning on this feature.