Merge branch 'dev/unified-mailboxes'

The Unified Mailboxes 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.

Uses can also customize the default unified mailboxes and create their
own.

The agent can be enabled or disalbed from KMail's Plugins settings like
other agents. By default the agent is off, however, if KMail detects that
user has more than one account set up it will ask the user if they want
to enable the feature. User is only ever asked once, the purpose is to
make the feature easier to discover without bothering users with only
one email account.

CHANGELOG: Unified mailboxes in KMail
FEATURE: 397649

Test Plan:

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: lueck, ngraham, mlaurent, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D15065