Moving most systray code out of winex11, and removing the need for a custom export in winemac. Maybe winemac could perhaps later use that new interface as well but I didn't want to mess with it too much.
-- v2: explorer: Remove now unnecessary wine_notify_icon support. winex11: Use the new SystrayDock driver interface. explorer: Use layered windows for the docked systray icons. win32u: Introduce new SystrayDock driver entry points. winemac: Use the new NotifyIcon user driver interface. win32u: Introduce a new NtUserSystemTrayCall message call. explorer: Hide the standalone systray window on close. explorer: Always set the systray icon tooltip text.