Set ActivationPolicy::Regular for macos in window example

[nicoburns]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

This fixes #3958. But possibly the default in Winit ought to change back, as it doesn't seem right that all users on macOS need to set this policy.

If I'm reading #3920 right, then it may be that this needs to be explicitly set when a rust binary is run standalone, but will be set automatically if the rust binary is wrapped in a macOS app package. I'm not quite sure how best to handle this. Perhaps we can detect which environment we're in somehow.

Unfortunately NSApplication::activationPolicy() seems to return 2 regardless of whether the setActivationPolicy() method is called, but keyboard events only work if it is.

[nicoburns]

@purajit Perhaps you might have a recommendation?

[purajit]

(Just a disclaimer I'm not a Mac developer, just an infra engineer who happened to look into
this issue, and if anyone has differing opinions they're probably more correct than mine)

I actually did wonder about what to do for the window example in that PR. As for the general
case of standalone non-bundled applications, I agree it doesn't make sense to have every
developer explicitly set ActivationPolicy. I see that NSRunningApplication provides bundle
information via bundleIdentifier, which should help us out here.

The value of this property will be nil if the application does not have an Info.plist.

I can PR this some time tomorrow, and hopefully that would mean the window example should
work as expected with no changes. If this is serious/urgent enough (like blocking a release), I'm
OK with having my PR reverted, and I can re-do it with the bundleIdentifier fix.

And apologies to anyone who hit this problem! Since a release hasn't yet been cut, I hope it didn't
impact too many people, and that this was caught early.

[madsmtm]

Thanks for submitting the issue and both PRs, I prefer the solution in #3961.