Fix 6250/Navbar: Classes being re-applied to every button in a navbar

@uGoMobi, @arschmitz:

Here is the fix for #6250 including test. This corrects the issue with `classes` being re-applied to every button element in the navbar.

Note:
- I'm using `data-nstest-role=button` in the test
- The buttons still get `ui-corner-all` applied, but this is not done in the navbar widget

I'm also not sure how to remove the previous commits here... I always seem to be adding to my previous commits :-(