Add Fluent menu and tooltip styles with high contrast support

[danielchalmers]

• The right-click menu, its submenus and the tooltips now use the Windows 11 style from DesktopClock: rounded corners, Segoe UI Variable (Segoe UI on Windows 10), check marks and the Size slider in the accent color, and a shadow on the menu. They follow Light and Dark now, where before they were always the old light style. The readings stay in Consolas.
• In Auto, the accent is lightened or darkened only as far as needed to reach 4.5:1 contrast against the widget (over a black or white desktop), the tooltip and the menu. Plum gets lighter in Dark, gold darker in Light.
• With high contrast on, everything uses the system colors: solid widget background, icons and graph in the text color, stale readings in the dimmed-text color, no menu shadow. Switching it on or off applies without a restart.
• The Ping address box matches the theme, title bar included, has its own Cut/Copy/Paste menu so that matches too, and its error red is readable in Dark.

Release notes: applies to everyone with no setting, and in Auto the icons can be a shade lighter or darker than the accent itself.