Improve how macdeployqt calls qmlimportscanner

Construct the argument list using "-rootPath", which
correctly handles multiple qmlDirs. Don't merge
qmlimportscanner stdout and stderr. Log stderr as
macdeployqt warnings.

Change-Id: I6f82f90f4e242c9aa246ab26ba3b9af88e1fd123
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>