Make the widget descend from HdyPreferencesRow directly for more freedom
on how to handle clicks and implement the nested rows, and instead embed
a HdyActionRow to implement the expander row itself, and a GtkListBox to
store the nested rows. Rows can be added directly to the expander and
will be stored in the nested list.

This drops the button in favor of clicking the expander row to hide the
nested list, use the hdy-expander-arrow-symbolic (similar to the
go-down-symbolic icon), and make the icon have a dim color when not
expanded and the accent color when expanded.

Fixes #90