Add user-configurable playback settings

* PlaybackManager has been updated to workaround limitiations in
  QtMultimedia
* PlaybackManager now sends the DeviceProfile to the server when
  determining the playback url. This makes the Jellyfin server send
  information back about transcoding.
* The DeviceProfile type has been changed from an QJsonObject into the
  DTO generated by the OpenAPI descripton.
* A settings page has been added on SailfishOS that allows the user to
  configure the PlaybackManager to their whishes.
* The DebugInfo page on SailfishOS now persists its settings (closes #8)

sailfish/qml/pages/settings/StreamingPage.qml

@@ -0,0 +1,67 @@
+/*
+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
+*/
+import QtQuick 2.6
+import Sailfish.Silica 1.0
+
+import nl.netsoj.chris.Jellyfin 1.0 as J
+
+import "../../components"
+import "../.."
+
+Page {
+    id: page
+
+    // The effective value will be restricted by ApplicationWindow.allowedOrientations
+    allowedOrientations: Orientation.All
+
+    SilicaFlickable {
+        anchors.fill: parent
+        contentHeight: content.height
+
+        Column {
+            id: content
+            width: parent.width
+
+            PageHeader {
+                title: qsTr("Streaming settings")
+            }
+
+            TextSwitch {
+                text: qsTr("Allow transcoding")
+                description: qsTr("If enabled, Sailfin may request the Jellyfin server " +
+                                  "to transcode media to a more suitable media format for this device. " +
+                                  "It is recommended to leave this enabled unless your server is weak.")
+                checked: appWindow.apiClient.settings.allowTranscoding
+                onCheckedChanged: appWindow.apiClient.settings.allowTranscoding = checked
+            }
+
+            Slider {
+                minimumValue: 0
+                maximumValue: 64 * 1024 * 1024
+                stepSize:  1024 * 128
+                valueText: qsTr("%1 mbps").arg((value / 1024 / 1024).toPrecision(4))
+                value: appWindow.apiClient.settings.maxStreamingBitRate
+                onDownChanged: if (!down) appWindow.apiClient.settings.maxStreamingBitRate = value
+                label: qsTr("Maximum streaming bitrate")
+                width: parent.width
+            }
+        }
+    }
+
+}