QbsProjectManager: Display compiler arguments quoted, if necessary.

On compile errors, file names with e.g. spaces are currently displayed
unquoted, which makes it very inconvenient to copy-paste the compiler
command line displayed in Creator to a shell for closer examination,
editing etc.

Change-Id: I935d8f0b5fa14c7a6f641dcfae659173882194b1
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>