Few fixes to get workable builds on VS2010.

- Fix for possible undefined behaviour of assignment operator ( discussion: https://forum.openmw.org/viewtopic.php?f=8&t=1975 );

- WIN32_LEAN_AND_MEAN definition is added to drop useless (for OMW) stuff from Windows.h (like Winsock, OLE headers, etc.);

- I received link-time conflict between OMW and SDL / double definition of WinMain() /, so third commit is about to fix that by defining SDL_MAIN_HANDLED.

By applying patches, described above, Windows user should get fully workable builds on all existent configurations.