Add PlaylistInfoItem.getDescription() and PlaylistInfoItemExtractor.getDescription()

[PeerTube] Implement the corresponding extractor method.
TODO: add tests

extractor/src/main/java/org/schabi/newpipe/extractor/playlist/PlaylistInfoItem.java

@@ -1,6 +1,7 @@
 package org.schabi.newpipe.extractor.playlist;
 
 import org.schabi.newpipe.extractor.InfoItem;
+import org.schabi.newpipe.extractor.stream.Description;
 
 import javax.annotation.Nullable;
 
@@ -13,6 +14,7 @@ public class PlaylistInfoItem extends InfoItem {
      * How many streams this playlist have
      */
     private long streamCount = 0;
+    private Description description;
     private PlaylistInfo.PlaylistType playlistType;
 
     public PlaylistInfoItem(final int serviceId, final String url, final String name) {
@@ -52,6 +54,14 @@ public class PlaylistInfoItem extends InfoItem {
         this.streamCount = streamCount;
     }
 
+    public Description getDescription() {
+        return description;
+    }
+
+    public void setDescription(final Description description) {
+        this.description = description;
+    }
+
     public PlaylistInfo.PlaylistType getPlaylistType() {
         return playlistType;
     }

extractor/src/main/java/org/schabi/newpipe/extractor/playlist/PlaylistInfoItemExtractor.java

@@ -2,6 +2,7 @@ package org.schabi.newpipe.extractor.playlist;
 
 import org.schabi.newpipe.extractor.InfoItemExtractor;
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
+import org.schabi.newpipe.extractor.stream.Description;
 
 import javax.annotation.Nonnull;
 
@@ -31,6 +32,16 @@ public interface PlaylistInfoItemExtractor extends InfoItemExtractor {
      */
     long getStreamCount() throws ParsingException;
 
+    /**
+     * Get the description of the playlist if there is any.
+     * Otherwise, an {@link Description#EMPTY_DESCRIPTION EMPTY_DESCRIPTION} is returned.
+     * @return the playlist's description
+     */
+    @Nonnull
+    default Description getDescription() throws ParsingException {
+        return Description.EMPTY_DESCRIPTION;
+    }
+
     /**
      * @return the type of this playlist, see {@link PlaylistInfo.PlaylistType} for a description
      *         of types. If not overridden always returns {@link PlaylistInfo.PlaylistType#NORMAL}.

extractor/src/main/java/org/schabi/newpipe/extractor/playlist/PlaylistInfoItemsCollector.java

@@ -41,6 +41,11 @@ public class PlaylistInfoItemsCollector
         } catch (final Exception e) {
             addError(e);
         }
+        try {
+            resultItem.setDescription(extractor.getDescription());
+        } catch (final Exception e) {
+            addError(e);
+        }
         try {
             resultItem.setPlaylistType(extractor.getPlaylistType());
         } catch (final Exception e) {

extractor/src/main/java/org/schabi/newpipe/extractor/services/peertube/extractors/PeertubePlaylistInfoItemExtractor.java

@@ -4,9 +4,12 @@ import com.grack.nanojson.JsonObject;
 
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
 import org.schabi.newpipe.extractor.playlist.PlaylistInfoItemExtractor;
+import org.schabi.newpipe.extractor.stream.Description;
 
 import javax.annotation.Nonnull;
 
+import static org.schabi.newpipe.extractor.utils.Utils.isNullOrEmpty;
+
 public class PeertubePlaylistInfoItemExtractor implements PlaylistInfoItemExtractor  {
 
     final JsonObject item;
@@ -54,4 +57,14 @@ public class PeertubePlaylistInfoItemExtractor implements PlaylistInfoItemExtrac
     public long getStreamCount() throws ParsingException {
         return item.getInt("videosLength");
     }
+
+    @Nonnull
+    @Override
+    public Description getDescription() throws ParsingException {
+        final String description = item.getString("description");
+        if (isNullOrEmpty(description)) {
+            return Description.EMPTY_DESCRIPTION;
+        }
+        return new Description(description, Description.PLAIN_TEXT);
+    }
 }