Calendar: Move MonthListModel into Calendar.h

Previously, we had two versions of MonthListModel for the AddEventDialog
and the DatePickerDialog. Now, a unified version is in the Calendar.h
file, which can be used easily by anyone. Since that model and the
MeridiemListModel weren't used anymore in the AddEventDialog, I have
also removed them from there.

Userland/Libraries/LibGUI/DatePicker.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2023, David Ganz <david.g.ganz@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <LibCore/DateTime.h>
+#include <LibGUI/Calendar.h>
+#include <LibGUI/Dialog.h>
+#include <LibGUI/Model.h>
+
+namespace GUI {
+
+class DatePicker : public Dialog {
+    C_OBJECT(DatePicker)
+
+public:
+    virtual ~DatePicker() override = default;
+
+    static Optional<Core::DateTime> show(Window* parent_window, String title, Core::DateTime focused_date = Core::DateTime::now());
+
+private:
+    explicit DatePicker(Window* parent_window, String const& title, Core::DateTime focused_date = Core::DateTime::now());
+
+    Core::DateTime m_selected_date;
+    RefPtr<ComboBox> m_month_box;
+    RefPtr<SpinBox> m_year_box;
+};
+
+}