Implemented category support

This implements category support similar to how the page module uses categories. The difference is that the default display `index.php?e=video` uses the category `system` for it's videos. Essentially the empty category codes that exist before this update get converted to the `system` category that gets added upon update. Top level categories get displayed on the default page `index.php?e=video` in combination to the `system` items and everything continues to function just like page list.

Updating will not cause any disruption to existing installation setups.

I am open to suggestions if you think a something should be changed.