Refactor|libcommon: Extracted high level savegame management into new C++ class 'SaveSlots'

At present the associated SaveInfos will continue to be owned by this
class. In the future these should be referenced from an index provided
by libdeng2's file system, which has none of the restrictions implicit
in the original games' slot-based mechanism.

Ultimately, SaveSlots will amount to little more than a mapping from
file paths into a finite set of 'slots'.

Todo: Cleanup