bugfix: forgot to mark split containers as split = true (+test) (Thanks szalik)

When the workspace layout (formerly orientation) was forced to change
due to a move command, the split container we created was not marked as
split = true, which caused tree_flatten() to errnously kill the contents
of it and thus one window ended up unmanaged.

Also, the logic in tree_flatten() was inverted due to commit de94f6da.

fixes #790

testcases/t/197-regression-move-vanish.t

@@ -0,0 +1,21 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+# Regression test: moving a window to the right out of a splitv container would
+# make it vanish.
+# Ticket: #790
+# Bug still in: 4.2-277-ga598544
+use i3test;
+
+my $ws = fresh_workspace;
+
+my $top = open_window;
+cmd 'split v';
+my $bottom = open_window;
+
+is_num_children($ws, 2, 'two windows on workspace level');
+
+cmd 'move right';
+
+is_num_children($ws, 2, 'still two windows on workspace level');
+
+done_testing;