Fix regression: Don’t add floating nodes twice when restoring layout (+testcase)

testcases/t/43-regress-floating-restart.t

@@ -0,0 +1,24 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Regression: floating windows are tiling after restarting, closing them crashes i3
+#
+use i3test tests => 1;
+use Time::HiRes qw(sleep);
+use X11::XCB qw(:all);
+
+my $tmp = get_unused_workspace();
+cmd "workspace $tmp";
+
+cmd 'open';
+cmd 'mode toggle';
+cmd 'restart';
+
+sleep 0.5;
+
+diag('Checking if i3 still lives');
+
+does_i3_live;
+
+my $ws = get_ws($tmp);
+diag('ws = ' . Dumper($ws));