Add support for inverse references navigation

[pcdavid]

• I have checked that this feature has not yet been suggested by someone else.

Expected behavior

Sirius Desktop supports the eInverse() AQL service in its intepreted expressions to navigate "backwards" on EMF references in the semantic model. This feature is used in a lot of existing VSMs, but we do not support it as it requires setting up AQL properly for that.

Cuttings backs

• Make the feature configurable (see below).
• Make the feature lazy (see below).

Potential side effects

The addition of an ECrossReferenceAdapter increases the memory footprint. If that is an issue, maybe this should be configurable (or the adapter installed lazyly on the first invocation of eInverse in a given project).

Describe alternatives you've considered

• Use one of the EcoreUtil.UsageCrossReferencer.findAll() on every call to CrossReferenceProvider#getInverseReferences. This would avoid the memory overhead, but replace it with a very costly operation on every invocation (rebuild a full UsageCrossReferencer each time).