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/jellyfin.h

@@ -1,13 +1,38 @@
+/*
+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_H
 #define JELLYFIN_H
 
 #include <QtQml>
 
-#include "JellyfinQt/jellyfinapiclient.h"
-#include "JellyfinQt/jellyfinapimodel.h"
-#include "JellyfinQt/jellyfinitem.h"
-#include "JellyfinQt/serverdiscoverymodel.h"
-#include "JellyfinQt/jellyfinplaybackmanager.h"
+#include "DTO/dto.h"
+#include "DTO/item.h"
+#include "DTO/mediastream.h"
+#include "DTO/namedguidpair.h"
+#include "DTO/types.h"
+#include "DTO/user.h"
+#include "DTO/userdata.h"
+
+#include "apiclient.h"
+#include "apimodel.h"
+#include "serverdiscoverymodel.h"
+#include "playbackmanager.h"
 
 namespace Jellyfin {
 void registerTypes(const char *uri = "nl.netsoj.chris.Jellyfin");