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/GDialog.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <LibGUI/GEventLoop.h>
+#include <LibGUI/GWindow.h>
+
+class GDialog : public GWindow {
+public:
+    enum ExecResult {
+        ExecOK = 0,
+        ExecCancel = 1,
+        ExecAborted = 2
+    };
+
+    virtual ~GDialog() override;
+
+    int exec();
+
+    int result() const { return m_result; }
+    void done(int result);
+
+    virtual const char* class_name() const override { return "GDialog"; }
+
+protected:
+    explicit GDialog(CObject* parent);
+
+private:
+    OwnPtr<GEventLoop> m_event_loop;
+    int m_result { ExecAborted };
+};