Put the progressbar and Pause / Cancel buttons into the headerbar

[Jeff Fortin Tam]

Sound Converter 4.0.4, as tested on Fedora 39, is very nice! I have one suggestion to make its UI a little bit more streamlined: instead of showing the progressbar and Cancel + Pause buttons in a statusbar at the bottom, why not reuse the headerbar instead? The headerbar already updates its title with the time estimate anyway.

You could:

* Temporarily hide the headerbar's buttons and put the Cancel / Pause buttons on its two extremities, setting one of them with either the suggested-action class or the destructive-action class as per https://developer.gnome.org/hig/patterns/controls/buttons.html#button-styles ; it's not like you can meaningfully use the other buttons during conversion anyway.

* Ideally, put the progressbar into the headerbar too, or if that's not possible maybe just packed directly below it (between the headerbar and the list view). Or maybe the progressbar is not necessary if you use HeaderBar's set_subtitle to set the progress % and time remaining estimation text into the subtitle property just below the "SoundConverter" title.