Indeterminate states for parent filter checkboxes

First commit is the behaviour change, the other 2 commits are just some renaming and organizing.

It might be easier to look at the first commit alone, then the other two separately (combined for convenience at: https://github.com/zanata/zanata-server/compare/150f46468edb3829e1988f1da4a66c55c073e416...9b9cfeaf9495a86da341c901380ebf589505c6c3 )