Fix the 'flagged' special use folder

...and sort folders with an unkown special use attribute as if it was a regular folder.
The 'flagged' folder (if it exists) now appears right beneath the 'all' and 'inbox' folders, feel free to question that order.

Also, the 'flagged' attribute has been left out of the guessSpecialRole() method on purpose, as it is probably a virtual folder and thus should be advertised by the server.

Fixes the second error message reported in #265 ;
@PoPoutdoor : can you test if your 'flagged' folders show up after this fix, and if it fixes #265 ?