hiro: Defer menubar updates on Windows to reduce flicker.

[Max833]

Menu bar updates are deferred to the event loop. A special case is made when no menu bar is visible, to ensure one is visible as early as possible during application startup.

Ported from:

https://github.com/higan-emu/higan/pull/146

Thanks to @jchv for that. This fixes bsnes' menubar as well.

[Max833]

I would like to do another pull request. Can someone review this one, @Screwtapello or @Kawa-oneechan? This would be really helpful, thank you. =)