PromptDialog: Port to SelectableText

SelectableLabel does not resize correctly due to using TextArea, if the text inside the dialog is dynamically changed. This is because TextArea disables resizing the implicitSizes after initialization.

Use the new SelectableText item instead, which works with dynamic resizing.

BUG:493581

---

Using following code as an example:

    Kirigami.PromptDialog {
        id: textPromptDialog
        title: "Title"
        subtitle: "empty"
        Component.onCompleted: {
            subtitle = "A long subtitle which should change the width of the PromptDialog <a href='https://google.com'>googel</a> <br> asdasda <br>asdasd"
        }
    }

Before	After