NWN: rename gui/main/main.cpp -> mainmenu.cpp

Here's my solution to one of the more annoying MSVC problems, namely intermediate object files overwriting each other if the .cpp files have the same name.

This is the easiest solution (the other one would be to split engines/nwn into multiple projects, which is not a very nice solution either)

There's another instance of 2 files with the same name in aurora/util.cpp and aurora/nwscript/util.cpp, and graphics/texture.cpp + graphics/aurora/texture.cpp, which can also be solved in two ways (renaming or splitting the project files)

Let me know what you think/which way you prefer