rhbz#1167250: Avoid X11Clipboard already being destroyed in constructor

...via SelectionManager::run's aKeep acquring and releasing getReference()

Change-Id: I18696d30c41ca9ae101261668cbf54cb0a6c45fc
(cherry picked from commit 94d2de0ba1f010be7acf0d00ce5b2ed4ed2b895b)
Reviewed-on: https://gerrit.libreoffice.org/13102
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>