Dont include dock clients in ewmh lists

http://standards.freedesktop.org/wm-spec/latest/ar01s03.html#idm140251368149456

The _NET_CLIENT_LIST property of the root window:

> These arrays contain all X Windows managed by the Window Manager.

Dock clients are not managed windows, so they should not be included in
_NET_CLIENT_LIST or _NET_CLIENT_LIST_STACKING.

include/con.h

@@ -37,6 +37,12 @@ void con_focus(Con *con);
  */
 bool con_is_leaf(Con *con);
 
+/**
+ * Returns true when this con is a leaf node with a managed X11 window (e.g.,
+ * excluding dock containers)
+ */
+bool con_has_managed_window(Con *con);
+
 /*
  * Returns true if a container should be considered split.
  *