Optimize QMediaPlaylistPrivate::readItems()

When reading playlist items from a file, pass them to the playlist
backend all at once rather than one by one. This might be faster
depending on the implementation.

Task-number: QTBUG-54849
Change-Id: I57acdc68604ee56fe5d7615ba0a72655e668443f
Reviewed-by: Christian Stromme <christian.stromme@qt.io>

src/multimedia/playback/qmediaplaylist.cpp

@@ -428,10 +428,12 @@ bool QMediaPlaylist::clear()
 
 bool QMediaPlaylistPrivate::readItems(QMediaPlaylistReader *reader)
 {
-    while (!reader->atEnd())
-        playlist()->addMedia(reader->readItem());
+    QList<QMediaContent> items;
 
-    return true;
+    while (!reader->atEnd())
+        items.append(reader->readItem());
+
+    return playlist()->addMedia(items);
 }
 
 bool QMediaPlaylistPrivate::writeItems(QMediaPlaylistWriter *writer)