Add optional line of secondary text to the notification

This feature adds an optional secondary line of text in a smaller font, displayed below the primary text. If no secondary text is provided, then the view behaves as it did before i.e. a single line of text is displayed, centred vertically in the view.

If secondary text is provided but no primary text is given, then the secondary text is treated as the primary text.

Currently, the activity indicator is not displayed is both primary and secondary text are specified.