Replace screenshots using libadwaita tooling

This replaces all the existing hand-crafted screenshots with screenshots that are generated using libadwaita's built-in tool. The README contains more details.

I've removed all the old screenshots and have not replaced all of them yet. Currently missing:

• Grid view
• List view
• Column view
• Selection mode
• Search
• Drop downs 🔥
• Sliders
• Notifications
• Info bars 🔥
• Progress bar 🔥
• Action dialog
• "Illustration" placeholder 🔥
• "Symbolic" placeholder 🔥
• Tooltip

I don't think we necessarily need all of these, but it would be good to have a bunch of them before the branch is merged.