Make it possible to mark a folder explicitly as sender folder

Summary:
CollectionStorageModel::isOutBoundFolder so far only checked the
MessageFolderAttribute to decide whether a folder was explicitly
marked as an outbound folder. But that meant that one could not
explicitly mark it as an inbound folder. This patch changes that
such that an explicit setting is properly honored. It is required
for people like me, who want to keep all mails in one folder. I
have my identity's sent folder (identity.fcc()) aliased to my inbox
folder. Without this patch, the sender/receiver column would show
receivers in the inbox, which is extremely confusing for me. With
this patch, I now properly see senders in my inbox again.

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D7627