Don't lazy populate all the children of library items

TODO

@@ -5,7 +5,6 @@
 - Copy to library, move to library
 - Edit tags in the playlist
 - Global shortcut keys
-- Make QSortFilterProxyModel on the library obey hasChildren()
 - Database versioning
 - Clicking play plays selected item
 

src/library.cpp

@@ -502,3 +502,11 @@ void Library::SetFilterText(const QString& text) {
   query_options_.filter = text;
   Reset();
 }
+
+bool Library::canFetchMore(const QModelIndex &parent) const {
+  if (!parent.isValid())
+    return false;
+
+  LibraryItem* item = IndexToItem(parent);
+  return !item->lazy_loaded;
+}

src/library.h

@@ -39,6 +39,7 @@ class Library : public SimpleTreeModel<LibraryItem> {
   Qt::ItemFlags flags(const QModelIndex& index) const;
   QStringList mimeTypes() const;
   QMimeData* mimeData(const QModelIndexList& indexes) const;
+  bool canFetchMore(const QModelIndex &parent) const;
 
  signals:
   void Error(const QString& message);