Implement selecting the next tiling/floating window (using "focus")

Also update documentation (manpage, userguide).
To make the code easier to read/write when checking if a client is
floating, introduce client_is_floating().

src/util.c

@@ -264,7 +264,7 @@ void unmap_workspace(xcb_connection_t *conn, Workspace *u_ws) {
 
         /* To find floating clients, we traverse the focus stack */
         SLIST_FOREACH(client, &(u_ws->focus_stack), focus_clients) {
-                if (client->floating <= FLOATING_USER_OFF)
+                if (!client_is_floating(client))
                         continue;
 
                 xcb_unmap_window(conn, client->frame);