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/handlers.c

@@ -466,7 +466,7 @@ static int handle_unmap_notify_event(xcb_unmap_notify_event_t *event) {
         return 1;
     }
 
-    tree_close(con, false, false);
+    tree_close(con, DONT_KILL_WINDOW, false);
     tree_render();
     x_push_changes(croot);
     return 1;