libdeng2|SavedSession: Defer SavedSession metadata caching post instantiation

As this is now a PackageFolder the filesystem may be in the process of
populating the folder contents, so reading and caching of the metadata
must be deferred.
libdeng2|SavedSession: Use absolute paths to SavedSessions in the "repository" index

Although the user's savegames will always exist in /home/savegames
we should require that all game saving is done here. For example,
Hexen uses a "base" savegame internally for storing the states of
visited maps in a hub for the current game session. This built-in
savegame should not be stored alongside the user's savegames.
UI|Client: Show the last modification time of a savegame in selection widgets

Also renamed some methods of SavedSession to conform with conventions
already established elsewhere.
libdeng2|SavedSession: Added convenient accessors to SavedSession::MapStateReader, cleanup
libcommon: Cleanup