MEDIUM: applet: centralize the call to si_applet_done() in the I/O handler

MEDIUM: applet: centralize the call to si_applet_done() in the I/O handler

It's much easier to centralize this call into the I/O handler than to
do it everywhere with the risk to miss it. Applets are not allowed to
unregister themselves anyway so their SI is still present and it is
possible to update all the context.