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Instead of using a KIO Slave for sending emails via SMTP, for which
KIO wasn't really designed, use a KSMTP library. This has no
functional or performance impacts, but improves maintainability
of our SMTP code and opens doors to further improvements (like
native Gmail authentication)

Differential Revision: https://phabricator.kde.org/D6879

Commit 351a0ce3, which switched from
QList to QVector, also switched a QSet to QVector: this gives as result
duplicate items for the connection results.

Since having duplicate items does not make sense, then switch the
internal storage back to QSet, manually converting it to the QVector
that is emitted as parameter of the finished signal.

We don't depend on KSmtp yet

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"