Support specifying the Qbs build directory as a command line option.

Also supports changing the default build directory ('.') using the
'preferences.defaultBuildDirectory' property.

Task-number: QBS-462
Change-Id: Id3d05d2f5b14197bd3af2e7f9d55bdd263809716
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>