Fixtures with attributes

I have modified excel-upload-download for attribute-support. But it depends on the fixture-remodelling. So, I am planning to do it as a separate PR once this re-modelling is good.
Other code in submodules that uses Fixtures is also modified, I am planning to create PRs for them once the main Fixture-modelling upgrade is done. 

I am thinking following upgrade plan for Fixtures.
* Have the fixture-remodel code reviewed. (going-on)
* Test that the modified XML-generation and fixture-filters work after above fixture-remodel. (tested)
* Once the fixture-remodel is finalized from above discussions, PR the fixture-excel-upload-download code. (wrote, but might need modifications based on above code-reviews)
* Create separate PRs for submodule fixture-dependencies. (depends on above)