LibWeb: Implement the CSS outline property :^)

...along with `outline-color`, `outline-style`, and `outline-width`.

This re-uses the existing border-painting code, which seems to work well
enough!

This replaces the previous code for drawing focus-outlines, with generic
outline painting for any elements that want it. Focus outlines are now
instead supported by this code in Default.css:

```css
:focus-visible {
    outline: auto;
}
```

Userland/Libraries/LibWeb/CSS/StyleProperties.h

@@ -68,6 +68,7 @@ public:
     Optional<CSS::Cursor> cursor() const;
     Optional<CSS::WhiteSpace> white_space() const;
     Optional<CSS::LineStyle> line_style(CSS::PropertyID) const;
+    Optional<CSS::OutlineStyle> outline_style() const;
     Vector<CSS::TextDecorationLine> text_decoration_line() const;
     Optional<CSS::TextDecorationStyle> text_decoration_style() const;
     Optional<CSS::TextTransform> text_transform() const;