Move platform configuration part from "system.h" to new header "PlatformConfig.h"
PlatformDefs.h: add include of <cstddef> for NULL and similar and for win32 add include of <WinNT.h> and <WinDef.h> for same win32 types (DWORD, HANDLE...) as defined in linux/PlatformDefs.h
Mass remove of "system.h" from headers; add "PlatformConfig.h", "PlatfromDefs.h" to some headers/.cpp; add "system.h" to some .cpp, replace some macros