Close empty workspaces if they're not visible.

Previously, we'd only close an empty workspace when we moved away
from it. Now we also close it when the last client exits, as long
as that workspace is not visible anymore.

testcases/t/178-regress-workspace-open.t

@@ -0,0 +1,22 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Tests if empty workspaces are closed when the last child
+# exits, as long as they're not empty.
+#
+use i3test;
+
+my $i3 = i3(get_socket_path());
+
+# Get a workspace and open a container
+my $ws = fresh_workspace;
+my $con = open_empty_con($i3);
+
+# Go to a second workspace, kill the container
+fresh_workspace;
+cmd "[con_id=\"$con\"] kill";
+
+# The first workspace should have been closed
+ok(!workspace_exists($ws), 'workspace closed');
+
+done_testing;