Fix multiselection behavior in playlist view

Do not (permanently) cache all entries of a playlist when the user has
made a selection. Only the selected songs are downloaded. When nothing
is selected, it will still download the whole playlist.

app/src/main/java/github/daneren2005/dsub/fragments/SelectDirectoryFragment.java

@@ -849,7 +849,7 @@ protected void executeOnValid(RecursiveLoader onValid) {
 	@Override
 	protected void downloadBackground(final boolean save) {
 		List<Entry> songs = getSelectedEntries();
-		if(playlistId != null) {
+		if(songs.isEmpty() && playlistId != null) {
 			songs = entries;
 		}