Browser+LibHTML: Add "Computed" styles to the DOM inspector

I though it would be nice to also show the style that the browser uses
to display an element.

In order to do that, in place of the styles table I've put a tab widget,
with tabs for both element and computed element styles.

Libraries/LibHTML/DOMComputedElementStyleModel.cpp

@@ -0,0 +1,50 @@
+#include "DOMComputedElementStyleModel.h"
+#include <LibHTML/CSS/PropertyID.h>
+#include <LibHTML/DOM/Document.h>
+#include <LibHTML/DOM/Element.h>
+
+DOMComputedElementStyleModel::DOMComputedElementStyleModel(const Element& element)
+    : m_element(element)
+{
+    if (element.layout_node() != nullptr && element.layout_node()->has_style()) {
+        element.layout_node()->style().for_each_property([&](auto property_id, auto& property_value) {
+            Value value;
+            value.name = CSS::string_from_property_id(property_id);
+            value.value = property_value.to_string();
+            m_values.append(value);
+        });
+    }
+}
+
+int DOMComputedElementStyleModel::row_count(const GModelIndex&) const
+{
+    return m_values.size();
+}
+
+String DOMComputedElementStyleModel::column_name(int column_index) const
+{
+    switch (column_index) {
+    case Column::PropertyName:
+        return "Name";
+    case Column::PropertyValue:
+        return "Value";
+    default:
+        ASSERT_NOT_REACHED();
+    }
+}
+GVariant DOMComputedElementStyleModel::data(const GModelIndex& index, Role role) const
+{
+    auto& value = m_values[index.row()];
+    if (role == Role::Display) {
+        if (index.column() == Column::PropertyName)
+            return value.name;
+        if (index.column() == Column::PropertyValue)
+            return value.value;
+    }
+    return {};
+}
+
+void DOMComputedElementStyleModel::update()
+{
+    did_update();
+}