Execute in background process libreoffice listener

* If listener is not started, script try to run it but can't continue processus
Provide a new constant about odt2pdf mangement : Don't remove odt source

* MAIN_ODT_AS_PDF_DEL_SOURCE by default disable
* If set then initial behaviour is conserved, odt file is removed
Merge pull request #1367 from camlafit/odt2pdf

Small improves on odt2pdf script