LibHTML: Rewrite inline and text layout

Inline layout is now done by LayoutBlock. Blocks with inline children
will split them into line boxes during layout.

A LayoutBlock can have zero or more LineBox objects. Each LineBox
represents one visual line.

A LineBox can have any number of LineBoxFragment children. A fragment
is an offset+length into a specific LayoutNode.

To paint a LayoutBlock with inline children, we walk its line boxes,
and walk their fragments, painting each fragment at a time by calling
LineBoxFragment::render(), which in turn calls the LayoutNode via
LayoutText::render_fragment(). Hit testing works similarly.

This is very incomplete and has many bugs, but should make it easier
for us to move forward with this code.

Libraries/LibHTML/Layout/LineBoxFragment.cpp

@@ -0,0 +1,12 @@
+#include <LibGUI/GPainter.h>
+#include <LibHTML/Layout/LayoutText.h>
+#include <LibHTML/Layout/LineBoxFragment.h>
+#include <LibHTML/RenderingContext.h>
+
+void LineBoxFragment::render(RenderingContext& context)
+{
+    if (layout_node().is_text()) {
+        auto& layout_text = static_cast<const LayoutText&>(layout_node());
+        layout_text.render_fragment(context, *this);
+    }
+}