QmakeNodes::addNewInformation: Be responsible subfolder nodes

And increase the priority for those too. Fixes the case:

a.pro
 b.pri
   test
 c.pri

Right click on test and add new file. In that case b.pri should be the
node to which the file is added.

Task-number: QTCREATORBUG-12272
Change-Id: I54d144861bc06545db8e71ed33513a0df971dae2
Reviewed-by: Eike Ziller <eike.ziller@digia.com>