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/GMessageBox.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <LibGUI/GDialog.h>
+
+class GMessageBox : public GDialog {
+public:
+    enum class Type {
+        None,
+        Information,
+        Warning,
+        Error,
+    };
+
+    explicit GMessageBox(const StringView& text, const StringView& title, Type type = Type::None, CObject* parent = nullptr);
+    virtual ~GMessageBox() override;
+
+    static void show(const StringView& text, const StringView& title, Type type = Type::None, CObject* parent = nullptr);
+
+    virtual const char* class_name() const override { return "GMessageBox"; }
+
+private:
+    void build();
+    RefPtr<GraphicsBitmap> icon() const;
+
+    String m_text;
+    Type m_type { Type::None };
+};