LibWeb: Paint the text selection :^)

Text selection currently works at the LayoutNode level. The root of the
layout tree has a LayoutRange selection() which in turn has two
LayoutPosition objects: start() and end().

A LayoutPosition is a LayoutNode + a text offset into that node.

We handle the selection painting in LayoutText::paint_fragment(), after
the normal painting is finished. The basic mechanism is that each
LayoutFragment is queried for its selection_rect(), and if a non-empty
rect is returned, we clip to it and paint the text once more.

Libraries/LibWeb/Layout/LineBoxFragment.h

@@ -28,6 +28,7 @@
 
 #include <AK/Weakable.h>
 #include <LibGfx/FloatRect.h>
+#include <LibGfx/Forward.h>
 #include <LibWeb/Forward.h>
 
 namespace Web {
@@ -68,6 +69,8 @@ public:
 
     int text_index_at(float x) const;
 
+    Gfx::FloatRect selection_rect(const Gfx::Font&) const;
+
 private:
     const LayoutNode& m_layout_node;
     int m_start { 0 };