Make a more sane infrastructure for included addons

[angryzor]

We have some repeated code in the attsrv api interface for dealing with addons already included in ticket levels which, combined with the special case in the addon component for this, made it clear that whether addons are selected is actually partially a derived property, and maybe shouldn't be also stored in a selected boolean for levels where they are already included.

We should think of a possible refactoring around this.