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.cpp

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2023, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "EventManager.h"
+#include <AK/JsonParser.h>
+#include <LibConfig/Client.h>
+#include <LibFileSystemAccessClient/Client.h>
+
+namespace Calendar {
+
+EventManager::EventManager()
+{
+}
+
+OwnPtr<EventManager> EventManager::create()
+{
+    return adopt_own(*new EventManager());
+}
+
+ErrorOr<void> EventManager::add_event(JsonObject event)
+{
+    TRY(m_events.append(move(event)));
+    set_dirty(true);
+    on_events_change();
+
+    return {};
+}
+
+void EventManager::set_events(JsonArray events)
+{
+    m_events = move(events);
+    on_events_change();
+}
+
+ErrorOr<void> EventManager::save(FileSystemAccessClient::File& file)
+{
+    set_filename(file.filename());
+    set_dirty(false);
+
+    auto stream = file.release_stream();
+    TRY(stream->write_some(m_events.to_deprecated_string().bytes()));
+    stream->close();
+
+    return {};
+}
+
+ErrorOr<void> EventManager::load_file(FileSystemAccessClient::File& file)
+{
+    set_filename(file.filename());
+    set_dirty(false);
+
+    auto content = TRY(file.stream().read_until_eof());
+    auto events = TRY(AK::JsonParser(content).parse());
+
+    set_events(events.as_array());
+
+    return {};
+}
+
+}