Group: Add background support

[melchoyce]

See #4900.

Now that the group block is pretty solid, let's explore adding background support:

(Please note that as this is an older mockup, many general UI elements are outdated.)

Some questions:

• We'd be reusing image settings from Cover.
• Instead of overlay color, I'm using opacity, so color and image can remain independent. Does this make sense?
• Should we use the new swap image flow for changing background images and include an image icon in the toolbar, instead of putting it in the sidebar?
• Is there any way to measure color contrast of text over an image?

[youknowriad]

This is a duplicate of #14744, can we consolidate?

[melchoyce]

Good catch, I'll close & move over.