DMN - Allow importing into the default namespace

[yesamer]

Currently, it is not possible to import a DMN file into another DMN file, specifying an empty string in the name attribute for the import. Implement the possibility to specify the name attribute as an empty string, with importing into the default namespace. That way, the imported decisions, etc. can be used without specifying the name prefix.

Conflict resolution - if the imported model contains an element with the same name as an element from the DMN model, the imported element is ignored. All other non-conflicting elements are imported properly. If there are multiple imports into the default namespace, and there are conflicts between the element names in the individual imports, the precedence of which element to import is specified by the order of the import definitions.

Acceptance criteria:

DMN engine can evaluate elements imported into the default namespace.
New tests are added to cover use cases by importing into the default namespace.
DMN editor can import a DMN model into the default namespace.