[Akonadi/PasteHelper] Add support for items linking

Summary:
When linking an item to a virtual collection via KAddressBook, `canPaste` check
fails because it's not aware of the context (i.e. action is `Link`, not `Copy`)
and checks only for `CanCreateItem` (which is not correct if we're linking).

This patch provides `Qt::DropAction action` parameter to `canPaste` method so
it is aware of action, and check for correct permissions based on that

Test Plan:
1) Link an item via KAddressBook using Drag'n'Drop to some virtual collection
(e.g. Google Contacts Resource)
2) The item gets actually linked, instead of silently ignoring

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28432