engine: internals supporting the facade migration

UndoStack push_pre_executed, FolderAddChild-based true folder moves,
NodeViewDeleteCommand delete-then-reconnect support, event emission
points for the facade event bus, and related internal adjustments.

engine/task/project/import/import.cpp

@@ -46,6 +46,11 @@ ProjectImportTask::ProjectImportTask(Folder *folder,
 	set_title(tr("Importing %n file(s)", nullptr, file_count_));
 }
 
+ProjectImportTask::~ProjectImportTask()
+{
+	delete command_;
+}
+
 const int &ProjectImportTask::get_file_count() const
 {
 	return file_count_;