- parse .qmake.cache, remove workaround for not doing it
- look for the mkspecs in all the places qmake would do
- use the passed in properties instead of QLibraryInfo to find the qt
  data dir
- fix loading of generic features from within os-specific ones with the
  same name
- add multiple inclusion guard for features (including what seems to be
  a bug in qmake ...)
- misc cleanups and beautifications