Fixed|libdeng2|ArchiveFeed: Handling corrupt source data

Even if the source archive was invalid and caused an exception when
reading, the feed was still added to the source file's deletion
audience. However, the exception caused the ArchiveFeed constructor
to be automatically undone, leaving an invalid pointer in the source
file's audience.
libdeng2|FS: Reinterpreting files

Added File::reinterpret() and a test for it in test_archive.

This allows taking an ordinary file or an already interpreted file
and reinterpreting it just like a feed would do at the time when
the file was originally created.
Merge remote-tracking branch 'remotes/origin/master' into savegame-refactor
libcommon: Updated .save package reinterpretation to use the new method

As of this commit, saving and loading of the game to .save packages
is once again possible.

Also fixed a bug in savedSessionByUserDescription() which meant that
the wrong folder was searched in for SavedSessions.