Enable automatic updates from GitHub

• Add temporary migration notifier for future updates from GitHub

• Enable automatic updates from GitHub

• Add temporary migration routine for future updates from GitHub

[+] Add support for Iceape-UXP
[+] Add hidden option to skip toolbar unhide

[±] Keep the button position when it's placed right before space, flexible space or separator

[+] Add option to limit the notification duration

[±] Change homepageURL to GitHub, update compatibilities

[±] Fix bug when removing button from toolbar in SeaMonkey

[±] Always run cleanup routine when URI scheme changed