Fix code style and errors

src/core/mainwindow.cpp

@@ -1321,7 +1321,7 @@ void MainWindow::AddToPlaylist(QAction *action) {
   }
 
   SongList songs;
-  for (PlaylistItemPtr item : items) {
+  for (const PlaylistItemPtr &item : items) {
     songs << item->Metadata();
   }
 
@@ -1587,7 +1587,7 @@ void MainWindow::EditTracks() {
 
 void MainWindow::EditTagDialogAccepted() {
 
-  for (PlaylistItemPtr item : edit_tag_dialog_->playlist_items()) {
+  for (const PlaylistItemPtr &item : edit_tag_dialog_->playlist_items()) {
     item->Reload();
   }
 
@@ -2256,7 +2256,7 @@ void MainWindow::AutoCompleteTags() {
 
 void MainWindow::AutoCompleteTagsAccepted() {
 
-  for (PlaylistItemPtr item : autocomplete_tag_items_) {
+  for (const PlaylistItemPtr &item : autocomplete_tag_items_) {
     item->Reload();
   }