Replace the desktop video path with OWE

The desktop background no longer plays videos. OWE owns video
backgrounds, and the shell layer stays empty behind one. The shell keeps
stills, which OWE hands back to it.

Remove the desktop video pause plumbing that only existed to stop an
unseen player: the lock, idle, and battery service lookups, the
per-output fullscreen check, the first-screen audio opt-in, and the audio
output in BackgroundVideo. The lock screen keeps its own silent playback.

Update the background tests, the manual, and the package note.

shell/Ui/BackgroundMedia.qml

@@ -7,11 +7,9 @@ Item {
   property string path: ""
   property int version: 0
   property bool playbackEnabled: true
-  property bool audioEnabled: false
-  // The desktop yields video playback to OWE while it runs, so two engines
-  // never decode the same file. The lock screen leaves this off and keeps its
-  // own playback.
-  property bool deferVideo: false
+  // The desktop turns this off because OWE owns video backgrounds. The lock
+  // screen leaves it on and keeps its own playback.
+  property bool videoEnabled: true
   // Bumped when the file behind an unchanged path may have been replaced.
   // Images cache-bust through version; a video is rebuilt, since FFmpeg
   // would read a query as part of the filename.
@@ -27,7 +25,7 @@ Item {
   // Both test the path directly: going through `video` lets a URL evaluate
   // against the stale flag and leak the wrong file for one pass.
   readonly property url imageUrl: path && !Util.isVideoPath(path) ? Util.fileUrl(path) + (version ? "?v=" + version : "") : ""
-  readonly property url videoUrl: path && Util.isVideoPath(path) ? Util.fileUrl(path) : ""
+  readonly property url videoUrl: path && Util.isVideoPath(path) && videoEnabled ? Util.fileUrl(path) : ""
 
   Loader {
     id: imageLoader
@@ -41,7 +39,7 @@ Item {
   Loader {
     id: videoLoader
     anchors.fill: parent
-    active: root.path !== "" && root.video && !root.reloading && !root.deferVideo
+    active: root.path !== "" && root.video && !root.reloading && root.videoEnabled
     source: "BackgroundVideo.qml"
   }
 
@@ -68,13 +66,6 @@ Item {
     when: videoLoader.item !== null
   }
 
-  Binding {
-    target: videoLoader.item
-    property: "audioEnabled"
-    value: root.audioEnabled
-    when: videoLoader.item !== null
-  }
-
   Component {
     id: imageComponent