Refactor|libcommon: Removed the now superfluous "base" save slot

As SaveSlots no longer own SavedSessions and the internal savegame
is managed independently, there is no need for "base" slot at all.

Also restructured some game session management logic, reducing some
unnecessary duplication and improved clarity.
libdeng2|SavedSession: Cleanup

Info no longer forces parsed key/value names to lowercase.