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/LibGUI/GBoxLayout.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <LibGUI/GLayout.h>
+#include <LibGUI/GWidget.h>
+
+class GBoxLayout final : public GLayout {
+public:
+    explicit GBoxLayout(Orientation);
+    virtual ~GBoxLayout() override;
+
+    Orientation orientation() const { return m_orientation; }
+
+    virtual void run(GWidget&) override;
+
+private:
+    Orientation m_orientation;
+};