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.

src/container.c

@@ -0,0 +1,43 @@
+/*
+ * 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"
+#include "log.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) {
+	int num_clients = 0;
+	Client *client;
+
+	if (con == NULL || con->mode == MODE_DEFAULT)
+		return MODE_DEFAULT;
+
+        if (!for_frame)
+                return con->mode;
+
+	CIRCLEQ_FOREACH(client, &(con->clients), clients)
+		num_clients++;
+
+	/* If the container contains only one client, mode is irrelevant */
+	if (num_clients == 1) {
+		DLOG("mode to default\n");
+		return MODE_DEFAULT;
+	}
+
+	return con->mode;
+}