import-task dependency management update

[hkuich]

• add ability to specify order of import. After adding attribute players in relations, one can now mess up imports by just taking the default ordering of:

1. independent attributes
2. entities
3. relations
4. nested-relations
5. appending attributes

because when having attributes as players, they can be added in steps 3, 4, and 5 - and it is no longer guaranteed that they are already present...

Solution:

1. adding independent attributes can and should always happen first
2. adding entities next can and should always happen second
3. next would be relations containing only entities
4. next would be relations containing entities and other relations
5. next would be appending attributes to existing entities or relations
6. finally all relations that have attributes as players
7. there should then be an option to have something be imported "afterwards" - for example, when one has a nested relation that contains attribute players, requiring that 6. is completed

[hkuich]

implementation finished - awaiting merge request