The current disc list implementation uses a GtkListBox as
the parent class. This brings some issues with it, as the
listbox widget has selectable, focusable rows that break
the keyboard navigation. The listbox rows also handle clicks
and add a strange effect to the disc list when clicking
outside the songs list.

Fix this by using GtkBox as the base class of DiscListBox,
and the keyboard and clicking issues are all gone. Remove a css
workaround as well.

https://bugzilla.gnome.org/show_bug.cgi?id=775929