QMake: Open .qmake.cache .stash and .conf in pro file editor

Change-Id: I8b0eebd20962b5f1d527e5fcb50d0781d76617e8
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>