Stop using GtkBox:padding, GtkBox:fill and GtkBox:expand

Stop using padding on the GtkBox children, instead do it on the child widgets themselves.

This is not supported in GTK4, so in preparation this needs to be tackled.

See Stop using GtkBox:padding, GtkBox:fill and GtkBox:expand in the Gtk docs and gnome-music!223 as an example.

Added points for getting rid of the deprecated use of xalign and yalign.