[MU4 Issue][Plugins] A "Qt.quit()" instruction in a plugin closes the entire MS4

Calling Qt.quit()in a plugin closes the entire MS4.

Although it is not longer recommended for plugin developers to use Qt.quit() to close their plugins, it can still be found in many plugins.

To be reproduced with this basic plugin:

import QtQuick 2.2
import MuseScore 3.0
import QtQuick.Controls 2.2
import QtQuick.Layouts 1.1 // was 1.3 in MS3, but 1.1 in Temperaments
import QtQuick.Window 2.3

MuseScore {
    menuPath: "Plugins.testBasicGUI"
    description: "This plugins shows a BasicGUI "
    version: "1.0"
    requiresScore: false
    pluginType: "dialog"
    id: mainWindow
    width : 300
    height : 300
    
    ColumnLayout {
        id: container
        anchors.margins: 5
        anchors.fill: parent
        spacing: 5
        CheckBox {
            text: "I'm the default component"
            checked: true
        }

        Button {
            text: qsTranslate("QPlatformTheme", "Close")
            Layout.alignment: Qt.AlignLeft
            onClicked: {
                //mainWindow.parent.Window.window.close();
                Qt.quit(); // as found in many-many plugins --> closes MS4
            }
        }
    }
}

Start by running the supplied basic MuseScore plugin and trigger the button calling Qt.quit(). Trace how plugin dialogs handle Qt.quit() and application shutdown. Done means the plugin can close without closing the entire MuseScore application, while preserving the expected behavior for existing plugins.