Initial version of Unified Mailboxes agent

The agent (which in reality is a Resource in order to be able to own
Collections) creates Inbox, Sent and Drafts virtual Collections and links
all Items from real special Collections into them. It listens for changes
in the source collections and updates the virtual Collections accordingly.

This is just an initial proof-of-concept, the plan is to allow users to
define custom unified mailboxes with the special Collections being only
the pre-defined defaults.

This agent will likely require some changes to KMail to allow better
interaction with the virtual Collections (e.g. deleting emails from the
unified mailbox should actually trigger Move to Trash action on the
source Collection).

The initial linking is very fast (even with a collection of ~80k emails)
but the brutal notification storm then triggers some asserts in ETMs
and freezes all apps for a decent amount of time due to ItemSerializer
parsing all the Items in the notification, so this will have to be
optimization in Akonadi.