Refactor|libdeng2|SavedSession|libcommon: Savegame system robustness improvements

SavedSessions are no longer copied. GameSessionWriter is now given a
repository path and produces a new session itself along with the .save
package.

Also began to remodel SavedSession into what will become a specialized
PackageFolder, removing the 1:1 relationship between it and the .save
file on disk.