This new implementation uses:
- The NSAppearanceCustomization protocol for retrieving the appearance
of the window, instead of using the application-wide
`-[NSApplication effectiveAppearance]`.
- Key-Value observing for observing the `effectiveAppearance` to compute
the `ThemeChanged` event, instead of using the undocumented
`AppleInterfaceThemeChangedNotification` notification.
This also fixes `WindowBuilder::with_theme` not having any effect, and
the conversion between `Theme` and `NSAppearance` is made a bit more
robust.
db2c97a995