Introduce two new callbacks for this: one when initializing the loading
of albums and another one on handling the loading of each album. Use the
first one to initialize and the second one to pulse the progress bar on
the Library panel.
Additionally use the text of the progress as status label instead of a
separate label widget.
As the documentation on the “locale” module states, the “gettext” module
should be used instead. Therefore adjust the localization calls to use
gettext instead.
Additionally fix one button label which did not use localization at all
and update the message catalogues.
