Make containers containing exactly one window behave like default containers

Starting from this commit, a borderless window will always be
borderless if it is the only window in a container. For example,
you can have Firefox borderless in a tabbed container and as soon
as the download manager or a viewer gets opened, the container
will be rendered like a normal tabbed container.

This solves the user-interface dilemma of borderless/1-px-border
windows inside stacked/tabbed containers, at least for this special
case. Thanks to Merovius for this suggestion.

include/container.h

@@ -0,0 +1,26 @@
+/*
+ * vim:ts=8:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ *
+ * © 2009 Michael Stapelberg and contributors
+ *
+ * See file LICENSE for license information.
+ *
+ */
+#include "data.h"
+
+#ifndef _CONTAINER_H
+#define _CONTAINER_H
+
+/**
+ * Returns the mode of the given container (or MODE_DEFAULT if a NULL pointer
+ * was passed in order to save a few explicit checks in other places). If
+ * for_frame was set to true, the special case of having exactly one client
+ * in a container is handled so that MODE_DEFAULT is returned. For some parts
+ * of the rendering, this is interesting, other parts need the real mode.
+ *
+ */
+int container_mode(Container *con, bool for_frame);
+
+#endif