Option for custom favicon in panel#337

It would be great to have the optional custom favicon in the panel back like in v2. Especially if you work with multiple Kirby-Instances.
And the black Kirby-Logo doesn’t look great in dark mode browsers with dark-gray Tabs ;)

This feature already in 3.6 and you can test it:
https://getkirby.com/releases/3.6/features/favicon