NSRunningApplication missing processIdentifier method

[ok-nick]

Hi, the NSRunningApplication class is missing the processIdentifier method. I'm assuming it's something to do with the pid_t type causing this method to not be auto-generated?

For now I'm going to use msg_send, but if there's anything that can be done to fix this issue I'd be happy to send a PR.

Edit: Does this look correct?

unsafe fn NSRunningApplication_processIdentifier(
    app: &icrate::AppKit::NSRunningApplication,
) -> std::os::raw::c_int {
    unsafe { msg_send![app, processIdentifier] }
}

[madsmtm]

Duplicate of #331, see that one for a few more details on why we don't do this type yet (because I want it to be libc::pid_t).

PRs welcome, though I can't guarantee it'll be easy to fix.