[builtin] fix activatewindow if directory path differs

After #5081 the built-in "activatewindow" does not work if you stay for example in the video window and you want to activate the same window with a different path.
This fixes the issue by comparing the directory path of the current active window with the given directory path of activatewindow.
The .empty() check is wrong, no?  Perfectly valid for a skin to want to go to the root.