Styles: make cards rounded, expand demo, new constants

[danirabbit]

Turns out everywhere we use ROUNDED is with CARD and the only places we don't use rounded with card are:

• in Files, Mail, and Photos because there's no easy way to crop overflow of images in GTK3, so this is not relevant and will be automatically fine in GTK4
• In AppCenter and Notifications indicator because we set a custom border radius

Basically there's no cases where we actually want square cards intentionally. So I think we can just make these round by default and deprecate the rounded class

[wpkelso]

Makes sense to me