fdo#82017: Rename 'Close without saving' button to 'Don't Save'.

It matches other applications both in Windows and OS X. Users expect to press
the 'D' key to activate this button. Also, it's shorter.

Change-Id: I08f80fd9dbfbc5e2a5dc69ff2f72d55b80ccda5a
Reviewed-on: https://gerrit.libreoffice.org/10685
Reviewed-by: Olivier Hallot <olivier.hallot@edx.srv.br>
Tested-by: Olivier Hallot <olivier.hallot@edx.srv.br>