Big C++ refractor

- Removed "jellyfin" prefix from files, as they are already in a
  directory named Jellyfin
- Split the former "jellyfinitem.{h,cpp}" into multiple files in the DTO
  directory, one for each class. The jellyfinitem files started to
  become enormous.
- Use forward declarations in headers instead of including files
  wherever possible.
- Updated copyright headers

core/include/JellyfinQt/websocket.h

@@ -0,0 +1,97 @@
+/*
+Sailfin: a Jellyfin client written using Qt
+Copyright (C) 2021 Chris Josten
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+#ifndef JELLYFIN_WEBSOCKET_H
+#define JELLYFIN_WEBSOCKET_H
+
+#include <QJsonDocument>
+#include <QJsonArray>
+#include <QJsonObject>
+#include <QJsonValue>
+#include <QDebug>
+#include <QObject>
+#include <QtGlobal>
+#include <QTimer>
+#include <QUuid>
+
+#include <QtWebSockets/QWebSocket>
+
+#include "apiclient.h"
+
+namespace Jellyfin {
+class ApiClient;
+
+namespace DTO {
+class UserData;
+}
+/**
+ * @brief Keeps a connection with the Jellyfin server to receive real time updates.
+ *
+ * This class will parse these messages and send them to ApiClient, which will emit signals for
+ * the interested classes.
+ */
+class WebSocket : public QObject {
+    Q_OBJECT
+public:
+    /**
+     * @brief WebSocket creates a webSocket for a Jellyfin server to handle real time updates.
+     * @param client The client to create the socket for.
+     *
+     * The socket will automatically set the ApiClient to its parent.
+     */
+    explicit WebSocket(ApiClient *client);
+    enum MessageType {
+        ForceKeepAlive,
+        KeepAlive,
+        UserDataChanged
+    };
+    Q_PROPERTY(QAbstractSocket::SocketState state READ state NOTIFY stateChanged)
+    Q_ENUM(MessageType)
+
+    QAbstractSocket::SocketState state() const {
+        return m_webSocket.state();
+    }
+public slots:
+    void open();
+private slots:
+    void textMessageReceived(const QString &message);
+    void onConnected();
+    void onDisconnected();
+
+    void sendKeepAlive();
+    void onWebsocketStateChanged(QAbstractSocket::SocketState newState) { emit stateChanged(newState); }
+signals:
+    void commandReceived(QString arts, QVariantMap args);
+    void stateChanged(QAbstractSocket::SocketState newState);
+
+protected:
+    ApiClient *m_apiClient;
+    QWebSocket m_webSocket;
+
+    QTimer m_keepAliveTimer;
+    QTimer m_retryTimer;
+    int m_reconnectAttempt = 0;
+
+
+    void setupKeepAlive(int data);
+    void sendMessage(MessageType type, QJsonValue data = QJsonValue());
+    QString generateMessageId();
+};
+}
+
+#endif // JELLYFIN_WEBSOCKET_H