ActionSheet customization, improved dark mode

[futuretap]

• Added imageView, accessoryView for action sheet cells;
• added cellLabelAlignment;
• tuned (iOS 13 dark mode) action sheet appearance matching UIAlertController:
  • opaque cancel background color
  • vibrant title/message label
  • adjusted background color (dynamic for light/dark in iOS 13)
  • adjusted blur effect on iOS 13

This PR squashes all commits of #285 into one clean commit.

[sberrevoets]

Thanks for this! I merged and then tweaked some stuff so I can keep track of what's going on in the future. I changed 2 things:

1. I changed the API and developer expectations around the image/accessory views and brought them more in line with how UITableViewCell works. The image view is now provided by the framework and the developer can only set an image on it and provide constraints. I also made the layout for these two a bit more explicit, messing with existing constraints on a view that wasn't created by the framework felt a bit odd.

2. Removed the cellLabelAlignment bit because the interaction felt a bit awkward because it had to be set for all actions at once and didn't apply to alert views. If this break some use case, let me know what it is and I can try to see if there's a nicer way to support this.

Let me know if the changes still support your use case and I will cut a new release.

[gereons]

I can't speak for @futuretap obviously, but I'd love to see this released :)

[futuretap]

Thanks, @sberrevoets, this looks good, I appreciate your changes to make the API cleaner. I personally don't need cellLabelAlignment, so I'm fine with removing it.