RFC: Addon callbacks for special-case stuff

This cleans up some of the `if (addon->Type() == ADDON_FOO)` over the codebase.  Most of it was there for PVR, but there's also service, repo, and skin-specific stuff.

It's not particularly nice to drop a bunch of includes (the skin ones being the worst) into the add-on classes, but it's better I think than having them elsewhere.

@negge, @xhaggi, @opdenkamp for comments on the PVR stuff.

@Montellese I'd appreciate your thoughts as well.

I'll add more doxy if it's accepted.  One thing to consider is the current special-casing that's left in CAddonDatabase::DisableAddon.  Reason it's there is due to the service+plugin+script in one add-on.  We need to know the add-on could be a specific type.  I wonder whether we need to seriously reconsider how we deal with multi-extension point add-ons.
:)