In each DesktopFileDir, store a list of desktop files for that
directory.  This speeds up opening desktop files by name because we can
skip statting in directories that we know don't have the file and also
speeds up _get_all() because we can avoid enumeration.

This also improves our support for dealing with names like
'kde4/kate.desktop' (equivalent to kde4-kate.desktop) since we find out
about all of these files are the start and don't need to guess about
which '-' to change to a '/'.  It also means that we can easily deal
with more than one level of such prefixes.

We use a file monitor to watch for changes, invalidating our lists when
we notice them.

https://bugzilla.gnome.org/show_bug.cgi?id=711520