LibWeb: Make box-shadow known throughout the CSS subsystem

This patch spreads box-shadows around:
- The Values important to box-shadows are stored in a BoxShadowData
  struct
- StyleProperties knows how to construct such a struct from a
  BoxShadowStyleValue and a Node knows how to ask for it
- CalculatedValues contain BoxShadowData and expose them

Userland/Libraries/LibWeb/CSS/ComputedValues.h

@@ -45,6 +45,13 @@ struct FlexBasisData {
     CSS::Length length {};
 };
 
+struct BoxShadowData {
+    CSS::Length offset_x {};
+    CSS::Length offset_y {};
+    CSS::Length blur_radius {};
+    Color color {};
+};
+
 class ComputedValues {
 public:
     CSS::Float float_() const { return m_noninherited.float_; }
@@ -64,6 +71,7 @@ public:
     Optional<float> flex_shrink_factor() const { return m_noninherited.flex_shrink_factor; }
     Optional<float> opacity() const { return m_noninherited.opacity; }
     CSS::JustifyContent justify_content() const { return m_noninherited.justify_content; }
+    Optional<BoxShadowData> box_shadow() const { return m_noninherited.box_shadow; }
     const CSS::Length& width() const { return m_noninherited.width; }
     const CSS::Length& min_width() const { return m_noninherited.min_width; }
     const CSS::Length& max_width() const { return m_noninherited.max_width; }
@@ -148,6 +156,7 @@ protected:
         CSS::Overflow overflow_x { InitialValues::overflow() };
         CSS::Overflow overflow_y { InitialValues::overflow() };
         Optional<float> opacity;
+        Optional<BoxShadowData> box_shadow {};
     } m_noninherited;
 };
 
@@ -197,6 +206,7 @@ public:
     void set_flex_shrink_factor(Optional<float> value) { m_noninherited.flex_shrink_factor = value; }
     void set_opacity(Optional<float> value) { m_noninherited.opacity = value; }
     void set_justify_content(CSS::JustifyContent value) { m_noninherited.justify_content = value; }
+    void set_box_shadow(Optional<BoxShadowData> value) { m_noninherited.box_shadow = move(value); }
 };
 
 }

Userland/Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -764,4 +764,18 @@ Optional<CSS::Repeat> StyleProperties::background_repeat_y() const
         return {};
     }
 }
+
+Optional<CSS::BoxShadowData> StyleProperties::box_shadow() const
+{
+    auto value_or_error = property(CSS::PropertyID::BoxShadow);
+    if (!value_or_error.has_value())
+        return {};
+
+    auto value = value_or_error.value();
+    if (!value->is_box_shadow())
+        return {};
+
+    auto box = verify_cast<CSS::BoxShadowStyleValue>(value.ptr());
+    return { { box->offset_x(), box->offset_y(), box->blur_radius(), box->color() } };
+}
 }

Userland/Libraries/LibWeb/CSS/StyleProperties.h

@@ -62,6 +62,7 @@ public:
     Optional<CSS::Overflow> overflow_y() const;
     Optional<CSS::Repeat> background_repeat_x() const;
     Optional<CSS::Repeat> background_repeat_y() const;
+    Optional<CSS::BoxShadowData> box_shadow() const;
 
     const Gfx::Font& font() const
     {