Allow multiple sorts indexes

[laulaz]

It could be great to allow results sorting with different sort indexes & orders (combining sort widgets queries).
For now, the sort is decided by the last sort widget.

[avoinea]

@laulaz I'm not sure if this is possible with ZCatalog. If it is, could you provide a query example?

[laulaz]

@avoinea it is possible (and working fine), for example : https://github.com/zopefoundation/Products.ZCatalog/blob/master/src/Products/ZCatalog/tests/test_catalog.py#L607