Add an API to get the natural width of each line in a QML Text element

In some situations we need to be able to align another QML element next to a line of text that is in a `Text` element. However, this is not possible to do for multi-line text because:

• `onLineLaidOut` gives you the full width of the Text element rather than the natural width of the current line
• `FontMetrics` and `TextMetrics` do not support elision or text wrapping
• `Text` does not make it possible to access the underlying `QTextLayout`

I would like to propose a patch that adds the `naturalTextWidth` and `naturalTextRect` as read-only properties to `QQuickTextLine` so that the user can actually know how the lines are really laid out.