Set and update TrayIcons

[aurium]

I see you have a src/UI/UITaskbar.js and a toolbar element, but as far as I could see, it has no system tray.

That is a useful UI for permanent notification and can replace windows for background services.

A SystemTray implementation needs:

• TrayIcon creation with linked app ownership.
• TrayIcon update for all features.
• TrayIcon destruction.
• TrayIcon features:
  • icon from any kind of image (from JPG to SVG), as a url or data url.
  • title to inform the user with a mouse-over.
  • drop-down menu, allowing to expose methods to the user.