Implement stack-limit for further defining how stack windows should look

Using this command, you can limit the amount of columns or rows for
a stacking container. This allows for better usage of screen estate
when using stacking containers with many clients.

Examples:
  i3-msg "stack-limit cols 2"
You will now have a stack window which has two columns of windows.

src/mainx.c

@@ -39,6 +39,7 @@
 #include "data.h"
 #include "debug.h"
 #include "handlers.h"
+#include "click.h"
 #include "i3.h"
 #include "layout.h"
 #include "queue.h"