Dropdown menu only working on 1 Level

This is reminiscent of Microsoft dropping its search capabilities out of Windows with the explanation:
Oh well, we didn't think anyone would need it.

Seems there is always the one thing in Drupal that someone mucks up and you have to spend time fixing with patches.

@imclean, many thanks for the code! Though I couldn't get a perfectly working "mobile menu". More precisely, if I get to a page on the last level, then the menu cannot be opened anymore by clicking it. I hope you will notice my comment and you'll be willing to help! Thanks in advance!