Panel: support solid color as `image` preview option#45

In addition to image previews or icons displayed in the figures of a pages section, it would be nice to have a colour option as well: reference a field that contains a colour value and use this colour to illustrate the page.

This might be helpful in cases where a site doesn’t use images but implements a colour scheme to categorise content.

Maybe gradients, too? 🙏

Maybe this should be merged with https://kirby.nolt.io/104