ICCCM 2.0 requires the window manager to manage a selection WM_Sn, where n
is the number of a screen managed. FVWM and xclipboard contain example
code for selection management. However, I have not detected the required
client message from xclipboard startup and have not tested FVWM.

Required targets for WM_Sn:
 Target      Type         Data
---------------------------------------------------------------------
 MULTIPLE    ATOM_PAIR    (see section 2.6.2)
 TARGETS     ATOM[]       List of targets supported
 TIMESTAMP   INTEGER      Server time used to acquire WM_Sn ownership
 VERSION     INTEGER[2]   {2, 0}