Reduce motion: NSWorkspace.accessibilityDisplayShouldReduceMotion and its change notification #31

Split out of #29 (the animation verbs, PR #30), because it is independent of them and already wrong without them: react-x11's desktopSettings().animations — the desktop's "reduce motion", which stops every looping style.animation — is read from XSETTINGS (Gtk/EnableAnimations) and nothing else, so on the Cocoa backend it reports true whatever the user set in System Settings › Accessibility › Display.

Two pieces:

• A query, e.g. accessibilityDisplayOptions() → { reduceMotion, reduceTransparency, increaseContrast, differentiateWithoutColor, invertColors } from NSWorkspace.sharedWorkspace's accessibilityDisplayShould… properties.
• The change, forwarded as a backend event (say accessibility-display-changed with the same shape) from NSWorkspaceAccessibilityDisplayOptionsDidChangeNotification, so a renderer can stop or start loops while the app runs, the way the XSETTINGS watch does on X11.

Mechanism only: what a renderer does with reduceMotion (react-x11: loops never start, transitions still run) stays in the renderer.