This is a rather large commit because I started working on these changes on the last evening at Akademy and on the train back home.

It mainly changes the plugin from a simple KABC::Resource to a KABC::ResourceABC, i.e. to a resource capable of having subresources.

This allows us to map all Akonadi collections with addressee data into one KResource plugin instance which means we can get rid of the rather awkward collection selection in the plugin's config.

Since adding KResource plugin, e.g. in KAddressBook, always shows the plugin's config page, rather than leaving it empty I decided to embed the Akonadi resource configuration KCM instead, conveniently enabling us to add/configure/remove contact data resources through KAddressBook, similar to how Mailody handles this for mail data resources.