Bugfix: `tree_split` should not split floating cons

Use the more robust test of `con_is_floating` for testing a container
for being floating.

fixes #1177

src/tree.c

@@ -387,7 +387,7 @@ void tree_close_con(kill_window_t kill_window) {
  *
  */
 void tree_split(Con *con, orientation_t orientation) {
-    if (con->type == CT_FLOATING_CON) {
+    if (con_is_floating(con)) {
         DLOG("Floating containers can't be split.\n");
         return;
     }

testcases/t/218-regress-floating-split.t

@@ -0,0 +1,35 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • http://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • http://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • http://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Make sure floating containers really can't be split.
+# Ticket: #1177
+# Bug still in: 4.7.2-81-g905440d
+use i3test;
+
+my $ws = fresh_workspace;
+my $window = open_floating_window;
+cmd "layout stacked";
+cmd "splitv";
+
+my $floating_con = get_ws($ws)->{floating_nodes}[0]->{nodes}[0];
+
+is(@{$floating_con->{nodes}}, 0, 'floating con is still a leaf');
+
+cmd 'floating disable';
+
+does_i3_live;
+
+done_testing;