[sources] synchronize source asynchronously when started from the UI and redirect to the forthcoming import log, for a better user experience (there is still a lot more to do there though). Closes #10468967

usr/bin/cubicweb-ctl usr/bin/
usr/lib/python2*/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl