add option to show command lines in build output

With "qbs build --show-command-lines" one can suppress
command descriptions and show actual command lines instead.

Change-Id: I98328254bad7fb85036abb0525c5ad7e734110c7
Task-number: QBS-615
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>