Refactor|libcommon: Continued dismantling p_saveg.cpp

High level logics for saving/loading game sessions are sufficiently
succinct to appear in place in g_game.cpp
libcommon: Cleanup
Refactor|libcommon|SaveInfo: Moved SV_HxHaveMapStateForSlot logic to SaveInfo::haveMapSession
libcommon: Cleanup
libcommon: Continued cleaning up savegame management
libcommon|SaveSlots: Identify logical save slots with unique string identifiers

Each save slot is referenced with a unique string identifier rather
than a numerical index. This greatly simplifies the SaveSlots API,
facilitates a better OO design and allows for additional slots to be
added at any time.
Refactor|libcommon: Renamed high level game save functions

Rather than constantly juggle the meaning and usage of 'save game',
'load savegame', we can easily disambiguate and clarify by swapping
the 'game' term with 'session'. Furthermore, "load game" already
has a higher level meaning (in Ring Zero).
Refactor|libcommon: Continued 'game' -> 'session' standardization
Documentation|libcommon|SaveSlots: SaveSlots apidoc fix (remove reference to special slots)

The API is no longer concerned with how the slots are named, or,
what their purpose is.
libcommon|SaveInfo|SaveSlots: Cleanup
Fixed|libcommon: Compiler warnings (missing braces around initializer)
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine into ui-multiplayer
libappfw|LabelWidget: Added special "separate" align mode for labels
Refactor|libappfw|Client: Fluent interface for widget Margins
libappfw|FoldPanelWidget: Adjusted fold indicator placement
libappfw|LabelWidget: Added overlay image drawing for labels

The "separate alignment" mode turned out to be infeasible, because
when used, the label should be manually sized rather than using
an expanding policy. This would have complicated the internal logic
of the widget too much.

Now labels can have a simple overlay image drawn on top.
UI|Client|libappfw: Adjusted fold panel indicators

Also, aligning the Reset buttons of the Renderer Appearance
sidebar vertically with the fold indicators.