Use proper WM_Sx registration for i3 (#4843)

i3's WM ownership is registered with the X Atom WM_S_S0 (for screen0),
instead of the correct WM_S0.  The relevant xcb-util API didn't
change, this is simply a bug in i3 that has always been there:
https://gitlab.freedesktop.org/xorg/lib/libxcb-util/-/blob/0.4.0/src/atoms.c#L65

testcases/t/545-i3-registration.t

@@ -0,0 +1,11 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Tests whether our WM registration is done with the correct WM_S0 selection.
+
+use i3test;
+
+my $x = X11::XCB::Connection->new;
+my $reply = $x->get_selection_owner($x->atom(name => 'WM_S0')->id);
+ok($reply, "registration successful");
+done_testing;