Calendar: Implement saving, loading, and displaying of calendars

The user can now save, load, and view calendars. A calendar is made up
of an array of events which are saved in a JSON file. In the future we
should implement the iCalendar standard instead of using a custom
format.

Userland/Applications/Calendar/EventCalendar.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2023, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "EventManager.h"
+#include <LibFileSystemAccessClient/Client.h>
+#include <LibGUI/Calendar.h>
+
+namespace Calendar {
+
+class EventCalendar final : public GUI::Calendar {
+    C_OBJECT(EventCalendar);
+
+public:
+    virtual ~EventCalendar() override = default;
+
+    EventManager& event_manager() const { return *m_event_manager; }
+
+private:
+    EventCalendar(Core::DateTime date_time = Core::DateTime::now(), Mode mode = Month);
+
+    ErrorOr<void> save(FileSystemAccessClient::File& file);
+    ErrorOr<void> load_file(FileSystemAccessClient::File& file);
+
+    virtual void paint_tile(GUI::Painter&, GUI::Calendar::Tile&, Gfx::IntRect&, int x_offset, int y_offset, int day_offset) override;
+
+    OwnPtr<EventManager> m_event_manager;
+};
+
+}