checkbox-gui:gui-engine:JobTreeNode: Remove extra verbosity

Those qDebug() calls really slow down the app startup
checkbox-gui:gui-engine.cpp: Improve logging

Remove the qDebug() call printing the app_blob
Print the job id in front of the restored job results when resuming a session
checkbox-gui:gui-engine.cpp: Use the current_job metadata value

When resuming a session and selecting rerun last job, just use the current_job
metadata instead of a complex call to NextRunJobIndex() as it fails to
properly restore the runlist index.

fixes https://bugs.launchpad.net/checkbox-gui/+bug/1362449
"automatic merge of lp:~sylvain-pineau/checkbox/fix-1362449/ by tarmac [r=][bug=1362449][author=sylvain-pineau]"