Adwaita: Fix link styling

• Fix broken visited link styling by avoiding invalid selector :link:visited - CSS :link and :visited are mutually exclusive.
• In link button, move %link from the child label to the parent button, so that the hover/active effect is applied within the whole button and the visited style is applied correctly.
• Remove invalid selectors :backdrop:backdrop.

Before - visited links don't have underline and don't use theme's $link_visited_color correctly:

After - the visited link: