Adwaita: always use link specific colors for link selector

link target uses $selected_bg_color and $selected_fg_color for the definition of some selectors (e.g. selected).

While $link_color and $link_visited_color are based on $selected_bg_color, this does not seem coherent.

Replacing $selected_*g_color in link target style with link colors defined in _colors.scss file.