Ensure that Xbase check expressions only use scalar variables

[istvanrath]

The task is to ensure (at editing time) that Xbase check expressions only access scalar pattern variables (EStrings, EInts, datatypes etc):

• to ensure correct incremental behaviour
• to avoid myserious xbase bugs for the interpreted case.

A solution to this issue obsoletes #312 and provides a minimal solution to #96.

[okrosa]

I commited a new validator which checks whether the variables in the check expression are only EDataTypes. I think we don't need more to handle this issue, so I set it to resolved.

[ujhelyiz]

Did you try to enter a variable containing an EObject using content assist?

[okrosa]

Yes, the content assist in an empty check constraint does not propose variables at all now, so it is not misleading. (However not 100% percent helpful at the same time, as the usable variables are missing as well.)

[istvanrath]

@okrosa please cherry pick into maintenance branch

[okrosa]

Cherry pick completed.

[istvanrath]

OK - works also with custom datatypes and enums! Well done!