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/LayoutInline.h

@@ -3,6 +3,7 @@
 #include <LibHTML/Layout/LayoutNode.h>
 
 class Element;
+class LayoutBlock;
 
 class LayoutInline : public LayoutNode {
 public:
@@ -14,5 +15,7 @@ public:
 
     virtual void layout() override;
 
+    virtual void split_into_lines(LayoutBlock& container);
+
 private:
 };