Add X clipboard persistence
Related to https://bugzilla.xfce.org/show_bug.cgi?id=16752
This allows to keep selected text in X clipboard (primary clipboard) until a new selection occurs, even if the first selection vanishes, independently of the content of the desktop environment clipboard (secondary clipboard).
This is the behavior of many software, like Firefox, Thunderbird, LyX, Xfce4-terminal, or Atom.