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/EventManager.h

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2023, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <AK/Noncopyable.h>
+#include <AK/OwnPtr.h>
+#include <LibFileSystemAccessClient/Client.h>
+#include <LibGUI/Window.h>
+
+namespace Calendar {
+
+class EventManager {
+    AK_MAKE_NONCOPYABLE(EventManager);
+    AK_MAKE_NONMOVABLE(EventManager);
+
+public:
+    static OwnPtr<EventManager> create();
+
+    String const& current_filename() const { return m_current_filename; }
+    void set_filename(String const& filename) { m_current_filename = filename; }
+    bool dirty() const { return m_dirty; }
+    void set_dirty(bool dirty) { m_dirty = dirty; }
+
+    ErrorOr<void> save(FileSystemAccessClient::File& file);
+    ErrorOr<void> load_file(FileSystemAccessClient::File& file);
+    ErrorOr<void> add_event(JsonObject);
+    void set_events(JsonArray events);
+    void clear() { m_events.clear(); }
+
+    JsonArray const& events() const { return m_events; }
+
+    Function<void()> on_events_change;
+
+private:
+    explicit EventManager();
+
+    JsonArray m_events;
+
+    String m_current_filename;
+    bool m_dirty { false };
+};
+
+}