Add a game-load gadget

[gajop]

A game-load gadget (one that would place units/features, etc.) seems like a common requirement to most LD games.
See: https://github.com/SpringCabal/LD34/blob/master/LuaRules/Gadgets/game_load.lua . Probably useful as it is.

[gajop]

Instead this should probably rely on s11n lib instead.
Scened must be made to output in a format useful with s11n.