Klipper: Do not insert secret data into history

Summary:
Password manager tools like Keepassx offer an option to clear
the clipboard/selection after some time, e.g. 10 seconds,
after the password was copied to the clipboard. This works fine,
but unfortunately the password isn't removed from Klipper's
history. This is a great security risk, which may make the use
of password managers impossible.

This patch changes Klipper::applyClipChanges(const QMimeData* clipData)
where clipboard data is inserted into history. If the data has an
additional mime type 'x-kde-passwordManagerHint' with the data 'secret',
it is not inserted into history.

For this to work as designed, password managers should add the
additional mime type 'x-kde-passwordManagerHint' to the mimeData
like following when copying a password to the clipboard:

QMimeData* mimeDataClipboard = new QMimeData();
const QString secretStr = "secret";
QByteArray secretBa = secretStr.toUtf8();
mimeDataClipboard->setText(password);  // this is the password to copy
mimeDataClipboard->setData("x-kde-passwordManagerHint", secretBa);
clipboard->setMimeData(mimeDataClipboard, QClipboard::Clipboard);

if (clipboard->supportsSelection()) {
    // we cannot use the same QMimeData, it's already owned by clipboard
    QMimeData* mimeDataSelection = new QMimeData();
    mimeDataSelection->setText(password); // this is the password to
copy
    mimeDataSelection->setData("x-kde-passwordManagerHint", secretBa);
    clipboard->setMimeData(mimeDataSelection, QClipboard::Selection);
}

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: dvratil, broulik, graesslin, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12539