Use a uniform way of getting the path to store books

If we can't put the books in ~/Books, we make a Books folder in the
XDG_DATA_HOME directory.  This provides consistency with the restricted
branch.
Add button on opening screen to switch to downloader
Move details about downloads to expandable widget
Don't show dialog at beginning in there aren't any books

This information is duplicated by the no-books label.  Give a nicer
title here on first run, though.
Set application name to be full reverse domain name

Because it would be too easy to just have it read from the manifest
file....