Argument for 'kill' for killing a specific window (now default) or the whole client (+test)

Use 'kill window' to kill a specific window (for example only one specific
popup), use 'kill client' to kill the whole application (or X11 connection to
be specific).

src/con.c

@@ -885,7 +885,7 @@ static void con_on_remove_child(Con *con) {
     int children = con_num_children(con);
     if (children == 0) {
         DLOG("Container empty, closing\n");
-        tree_close(con, false, false);
+        tree_close(con, DONT_KILL_WINDOW, false);
         return;
     }
 }