libdeng2: Extracted game::Session base class from common::GameSession

The game::Session base class grants the engine some limited visibility
of and access to the logical game side session and the associated save
game functionality.

The SavedSession index is owned (shared) by this base class, rather
than the ResourceSystem (the index functionality will me merged into
the file system at some point), allowing the removal of the temporary
Base API entrypoint for this.
Fixed|libcommon|GameRuleset: Translation of boolean game rules, "fast" etc...
Fixed|Renderer|Client: Drawing of sky domes with multiple layers

IssueID #1736