EntryStore: Add methods for further slicing entry list
- children() - get immediate children of entry
- parent($entry) - get immediate parent of entry
- days_for($container) - get days in a month or year
- months_for($year) - get months contained in a year
- basename($entry) - get basename of an entry
These are used in App::WRT::year() and App::WRT::month(), replacing
some dir_list() invocations and at least somewhat simplifying the code.