Small fixes, but that have an impact of how things are visualized.

* Move "not tagged" elements to the end. We sort the rest (when they have tags), but keep untagged elements (the category) at the end of the view, since the user had "less" interest on them.

* Break the law again of "dirs first". If we have on a folder only a tagged file (and is a file) and the rest are folders, that file goes first, since it was tagged.

* If we are on the same category, dirs go first. Here we make them citizens of first class.

* Commas instead of spaces when building categories for tags, they make it more readable and less error-prone (we can have tags "a", "dog" and "a dog" as tags, right?, so spaces here make the thing a bit hard to distinguish, I have two tags "a", "dog", or the tag "a dog") ;)

* Return an empty string when the item is not tagged at the method tagsForIndex. This way we always show untagged items the latest ones very easily (without nasty string checks). We will let the categorizer put a fancy name for them, if the returned string was empty.