defer certain events on macOS to avoid re-entrant calls

[httnn]

previously, calling window.focus() would cause a crash in some scenarios, because certain members of WindowState were borrowed mutably while already being borrowed. this PR fixes the issue by deferring window focus events to be triggered before calling WindowHandler:on_frame().

[dathinaios]

I don't have a macOS machine to test this on however, and I'm not very familiar with macOS's APIs in general yet, so I think another pair of eyes would be good to double check this. 🙂

Tested on MacOS Ventura using Reaper, and it is working great 🙂👍