Fix shortcut not working under non English locale

This change wraps calls to QAction::shortcut() done in QML files to use the portable representation of QKeySequence.

The QKeySequence to QVariant operator return a string representation using the native format not the portable format. E.g., the sequence "Space", will be "Espace" under French locale. The 'shortcut' attribute of QtQuick Action is stored as QVariant. In qml file, when the QAction::shortcut() value (a QKeySequence) is affected to a QtQuick Action shortcut, the previous operator is used and the shortcut value may be bogus depending on translation.

// A QAction* with the Qt::Key_Space shortcut
property var playPauseAction: ElisaApplication.action("Play-Pause")
Action {
    // - playPauseAction.shortcut returns QKeySequence(Qt::Key_Space).
    // - QKeySequence(Qt::Key_Space) is transform into a QVariant that
    //        contains native text "Espace".
    // - this QVariant is affected to shortcut: broken.
    shortcut: playPauseAction.shortcut
    onTriggered: ElisaApplication.audioControl.playPause()

BUG: 424890 BUG: 410110

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