This fixes an issue where pressing the red close button in a window or
the "x" button on a tab couldn't differentiate and would always close
the tab or close the window (depending on tab counts).
It seems like in both cases, AppKit triggers the `windowShouldClose`
delegate method on the controller, but for the close window case it
triggers this on ALL the windows in the group, not just the one
that was clicked.
I implemented a kind of silly coordinator that debounces
`windowShouldClose` calls over 100ms and uses that to differentiate
between the two cases.
This is recommended for macOS Tahoe and all standard menu items now have
associated images. This makes our app look more polished and native for
macOS Tahoe.
For icon choice, I tried to copy other native macOS apps as much as
possible, mostly from Xcode. It looks like a lot of apps aren't updated
yet. I'm absolutely open to suggestions for better icons but I think
these are a good starting point.
One menu change is I moved "reset font size" above "increase font size"
which better matches other apps (e.g. Terminal.app).
