Backport for X11 Display Server fixes on 3.2 branch.
1. Implement SAVE_TARGETS mechanism
Allows sending the clipboard content to the clipboard manager on exit to
keep the content when using a clipboard manager that doesn't
automatically makes a backup when copying.
MULTIPLE selection mechanism also had to be implemented, because in this
case, the clipboard manager might request multiple selection targets at
once.
Known use case: Ubuntu with XFCE4
2. Implement INCR mechanism
Allows pasting from x11 clipboard to receive data incrementally, which
is required when handling data size > 256KB.
de4a1d09b0
