LibWeb: Convert images to common AbstractImageStyleValue base

This commit moves both the ImageStyleValue and LinearGradientStyleValue
to a common base class of AbstractImageStyleValue. This abstracts
getting the natural_width/height, loading/resolving, and painting
the image.

Now for 'free' you get:

 - Linear gradients working with the various background sizing/repeat
   properties.
 - Linear gradients working as list-markers :^) -- best feature ever!

P.s. This commit is a little large as it's tricky to make this change
incrementally without breaking things.

Userland/Libraries/LibWeb/Layout/ListItemMarkerBox.h

@@ -16,7 +16,6 @@ public:
     explicit ListItemMarkerBox(DOM::Document&, CSS::ListStyleType, size_t index, NonnullRefPtr<CSS::StyleProperties>);
     virtual ~ListItemMarkerBox() override;
 
-    Gfx::Bitmap const* list_style_image_bitmap() const;
     String const& text() const { return m_text; }
 
     virtual RefPtr<Painting::Paintable> create_paintable() const override;