Validate matched containers for "kill" command correctly.

We now execute the validations when "kill" is executed even if match
criteria are used. This prevents users from killing workspace containers,
which instead kills all clients (as before when not using criteria).

fixes #1761

include/con.h

@@ -30,6 +30,12 @@ Con *con_new(Con *parent, i3Window *window);
  */
 void con_focus(Con *con);
 
+/**
+ * Closes the given container.
+ *
+ */
+void con_close(Con *con, kill_window_t kill_window);
+
 /**
  * Returns true when this node is a leaf node (has no children)
  *