Adds a couple of hashes in the App::WRT::EntryStore::new() constructor
which stash arrays of properties for each entry, and arrays of entries
for each property, as File::Find works its way through the $entry_dir.

Uses props_for() in App::WRT::entry_topic_list() instead of dir_list().
This is maybe a tiny performance gain, but also probably more hassle
than it's worth unless other features are going to be built on these
methods (or other code that calls dir_list() simplified).

This also moves a chunk of logic out of EntryStore::all() and into the
constructor, and directly accesses $self->{source_files} in a couple of
methods instead of tossing around copies of the array.

I'm kind of iffy on whether any of this is worth doing.