Addon manager improvements

Bikeshedding PR. I'm sure everyone can find something you HATE;)

* Merges enabled/disabled into to one node and flatten. Nobody will several hundreds of addons installed, so a flat list is easier to navigate. Uninstallable- and addons user shouldn't uninstall or disable, are hidden.

* For whatever reason people want to see those addons, a 'system addons' node is added. 

* Hides all support addon types users shouldn't install from repository.

and other fixes