LibHTML: Build some foundation for text selection

Add LayoutPosition and LayoutRange classes. The layout tree root node
now has a selection() LayoutRange. It's essentially a start and end
LayoutPosition.

A LayoutPosition is a LayoutNode, and an optional index into that node.
The index is only relevant for text nodes, where it's the character
index into the rendered text.

HtmlView now updates the selection start/end of the LayoutDocument when
clicking and dragging with the left mouse button.

We don't paint the selection yet, and there's no way to copy what's
selected. It only exists as a LayoutRange.

Libraries/LibHTML/Layout/LayoutPosition.h

@@ -0,0 +1,56 @@
+#pragma once
+
+#include <AK/RefPtr.h>
+
+class LayoutNode;
+
+struct LayoutPosition {
+    bool operator>=(const LayoutPosition& other) const
+    {
+        if (layout_node == other.layout_node)
+            return index_in_node >= other.index_in_node;
+
+        // FIXME: Implement.
+        return true;
+    }
+
+    bool operator<=(const LayoutPosition& other) const
+    {
+        if (layout_node == other.layout_node)
+            return index_in_node <= other.index_in_node;
+
+        // FIXME: Implement.
+        return false;
+    }
+
+    RefPtr<LayoutNode> layout_node;
+    int index_in_node { 0 };
+};
+
+class LayoutRange {
+public:
+    LayoutRange() {}
+    LayoutRange(const LayoutPosition& start, const LayoutPosition& end)
+        : m_start(start)
+        , m_end(end)
+    {
+    }
+
+    bool is_valid() const { return m_start.layout_node && m_end.layout_node; }
+
+    void set(const LayoutPosition& start, const LayoutPosition& end)
+    {
+        m_start = start;
+        m_end = end;
+    }
+
+    void set_start(const LayoutPosition& start) { m_start = start; }
+    void set_end(const LayoutPosition& end) { m_end = end; }
+
+    const LayoutPosition& start() const { return m_start; }
+    const LayoutPosition& end() const { return m_end; }
+
+private:
+    LayoutPosition m_start;
+    LayoutPosition m_end;
+};