hide includes from within feature files

hide includes from within feature files

this fixes the recent appearance of qt_webkit_version.pri in all qt 4.7
based projects.

in fact, the patch does somewhat more: it makes the evaluator provide
the information to the higher layers which is necessary for a truly
hierarchical display of includes. TBD later.

Reviewed-by: dt
Task-number: QTCREATORBUG-1590