allow to set filters in designspace applying to all masters

currently if one wants to apply a filter to all masters (e.g. "Decompose Components"), one has to duplicate it in each master UFO's lib.plist.
Ideally one should be able to set this common filter once in the Designspace element and ufo2ft would treat it as applying to all the sources in the DS.

Start by tracing how ufo2ft reads the Designspace element and how filters are currently read from each master UFO's lib.plist. Confirm the expected behavior with a small Designspace containing one common filter and multiple sources; done means the common filter applies to every source without duplicating it in each UFO.

Written by the indexing model from the issue text.