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/jsonhelper.h

@@ -1,3 +1,21 @@
+/*
+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 JSON_SERIALIZER_H
 #define JSON_SERIALIZER_H