Libraries: Create top level directory for libraries.

Things were getting a little crowded in the project root, so this patch
moves the Lib*/ directories into Libraries/.

Libraries/LibHTML/Layout/LayoutBlock.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibHTML/Layout/LayoutNode.h>
+
+class Element;
+
+class LayoutBlock : public LayoutNode {
+public:
+    LayoutBlock(const Node&, const StyledNode&);
+    virtual ~LayoutBlock() override;
+
+    virtual const char* class_name() const override { return "LayoutBlock"; }
+
+    virtual void layout() override;
+
+private:
+    virtual bool is_block() const override { return true; }
+
+    void compute_width();
+    void compute_height();
+};