Recognise inbox collection from all resources

This fixes filtering on non-IMAP resources as the current implementation
only considers for filtering folders with server-side name "INBOX" or
the default Inbox collection for the Akonadi instance. If the resource
uses a different server-side identifier for its Inbox folder it will not
get filtered.

The updated implementation will consider all Inbox-type folders for all
resources that have registered such folders.

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D3181