Fix favicon links

Good browsers need "icon" and (hopefully) can use SVG.
Non-standards-compliant browsers need "shortcut icon" and can't use SVG.
Minor text changes and reformatting
Fix downloads link

GitHub deprecated "downloads" and made "releases".
Update screenshots

* GTK3.
* Ditch thumbs, and ditch big fullscreen image.
* Increase size so that smallest image is full-size.