Add VVV_exit()

This is just a function that calls the cleanup() in main.cpp, as well as
calls exit().

I would have liked to use SDL_ExitProcess() here, because that function
has ifdefs for different runtime environments. But alas, it's an
internal function and isn't exported. Ah well; exit() seems to be fine
anyway.

desktop_version/src/Exit.h

@@ -0,0 +1,6 @@
+#ifndef VVV_EXIT_H
+#define VVV_EXIT_H
+
+void VVV_exit(const int exit_code);
+
+#endif /* VVV_EXIT_H */