Skip nested Feed when calling getNextUnreadFeed()

This function is only called when using "Mark all as read".
So every time, this function gets called right now, all nested
categories get marked as read as well, so we don't want to jump to them.
Instead we want to the next category with same or higher tier.
Merge pull request #379 from Joschasa/getNextUnreadFeed_skip_nested

Skip nested Cats/Feeds when calling getNextUnreadFeed()