Touching nsXULAppAPI.h causes half of the codebase to be recompiled which is bad.

A cursory grep shows at least 47 files including the header but not using any of the functions in there (they might be using definitions though).

Also most of the files including it only want XRE_GetProcessType() or XRE_IsParentProcess() so it should be easy to move those out to a smaller header for code that doesn't need to call those functions from outside of libxul.