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/src/DTO/mediastream.cpp

@@ -0,0 +1,43 @@
+/*
+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
+*/
+#include "JellyfinQt/DTO/mediastream.h"
+
+namespace Jellyfin {
+namespace DTO {
+
+MediaStream::MediaStream(QObject *parent) : JsonSerializable (parent) {}
+MediaStream::MediaStream(const MediaStream &other)
+    : JsonSerializable (other.parent()),
+      m_codec(other.m_codec),
+      m_codecTag(other.m_codecTag),
+      m_language(other.m_language),
+      m_displayTitle(other.m_displayTitle),
+      m_type(other.m_type),
+      m_index(other.m_index){
+}
+bool MediaStream::operator==(const MediaStream &other) {
+    // displayTitle is explicitly left out, since it's generated based on other properties
+    // in the Jellyfin source code.
+    return m_codec == other.m_codec && m_codecTag == other.m_codecTag
+            && m_language == other.m_language && m_type == other.m_type
+            && m_index == other.m_index;
+}
+
+} // NS DTO
+} // NS Jellyfin