QString::isNull() vs. isEmpty() discrimination is a time bomb ...

the root cause is in the API of parsedProFile(): it discerns
isNull() and isEmpty(), which is of course a time bomb.
but in this case the right solution is not trying to parse
the empty string in the first place.

Done-with: Roberto Raggi

This simple change will save about 40mb of memory when indexing Qt Creator!

e.g. DEFINES+=bla on the qmakestep command line now work

Task-Nr: QTCREATORBUG-2091

 * Fix all warnings about non-explicit constructors except for those
   in 3rdparty code.

That is tell the user that those files need not be added to any project,
and show "<Implicitly Add>" for them on the last wizard page.
This fixes Add/New for the QML/OTHER_FILES folder structure, except for
the virtual folder itself.

Reviewed-By: Jarek Kobus <jaroslaw.kobus@nokia.com>