Allow to easily retrieve the exact binary used in a project. This will be used by the QMake Manager.

CC'ing kdevelop-devel to get some input on a question wrt. to this change: Would it be better to offer something like QString queryQMake(const QString& var, KDevelop::IProject*) which would basically run qmake -query var and return the result?

I need a way in the qmake manager to run qmake -query var but am not 100% sure which of the two API's is "better" and I'd like to not hardcode the qmake group config-key in two places...