Sys_GetScanTable() and MapKey() are only used by the Windows backend
or the Windows-only tools.
Rename to Win_GetScanTable() and move MapKey() as Win_MapKey() to
win_input.cpp.
Don't include the lazy precompiled.h everywhere, only what's
required for the compilation unit.
platform.h needs to be included instead to provide all essential
defines and types.
All includes use the relative path to the neo or the game
specific root.
Move all idlib related includes from idlib/Lib.h to precompiled.h.
precompiled.h still exists for the MFC stuff in tools/.
Add some missing header guards.
