Convert current statistics information into a lua gadget

There's no reason for statistics information being hard-coded into the engine instead of being a gadget.
Examples are:
PlayerStatistics.h
TeamController.h
SkirmishAIBase.h

Start by inspecting the statistics-related code in PlayerStatistics.h, TeamController.h, and SkirmishAIBase.h, then trace how the engine currently exposes that information. Determine the scope of the Lua gadget replacement and its expected behavior from the surrounding engine code. Done means the statistics information is no longer hard-coded in the engine and is provided through a Lua gadget.

Written by the indexing model from the issue text.