Summary:
When current folder is a virtual collection, the Items in it may belong
to multiple different resources and as such have different Trash folders.
This change makes KMTrashMsgCommand aware of this fact and to handle
this case correctly by resolving parent collections of all selected
messages.

Test Plan:
Tested against the Unified Mailboxes Agent (currently loving in
dev/unified-mailboxes branch) which has virtual collections.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14465