detail: allow adding to playlist in playlist

The playback view can still add to a playlist from a playlist, so I
have to implement this navigation route no matter what.

app/src/main/java/org/oxycblt/auxio/detail/PlaylistDetailFragment.kt

@@ -328,7 +328,11 @@ class PlaylistDetailFragment :
                     logD("Deleting ${decision.playlist}")
                     PlaylistDetailFragmentDirections.deletePlaylist(decision.playlist.uid)
                 }
-                is PlaylistDecision.Add,
+                is PlaylistDecision.Add -> {
+                    logD("Adding ${decision.songs.size} songs to a playlist")
+                    PlaylistDetailFragmentDirections.addToPlaylist(
+                        decision.songs.map { it.uid }.toTypedArray())
+                }
                 is PlaylistDecision.New -> error("Unexpected playlist decision $decision")
             }
         findNavController().navigateSafe(directions)

app/src/main/res/navigation/inner.xml

@@ -379,6 +379,9 @@
         <action
             android:id="@+id/delete_playlist"
             app:destination="@id/delete_playlist_dialog" />
+        <action
+            android:id="@+id/add_to_playlist"
+            app:destination="@id/add_to_playlist_dialog" />
         <action
             android:id="@+id/play_from_artist"
             app:destination="@id/play_from_artist_dialog" />