Add ability to hide "category" level

[GoogleCodeExporter]

I'd like to have an ability to hide "category" level. In simpliest case 
possible I'd like to have plain flat list of configuration, no any groups or 
folding. I set "Flat tree" but there is still 2 levels - category and list of 
configurations. I wish I could hide the category level.

Even when I have "Tree with types" set, I doubt I need "Uncategorized" level at 
all provided there are no other categories. So it's either should only be seen 
when there are other categories, or a button to toggle regardless of any 
categories defined.

Original issue reported on code.google.com by paxusvl on 25 Sep 2013 at 6:42