[Data import] Allow batch import of nodes and edges in order to append an existing graph

Feature description

Allow the user to import a set of nodes and edges via a JSON file and add the set to an existing graph. Add menu item 'append graph' to the 'File' sub-menu.

Use cases

Select 'File' -> 'Append graph' -> choose a JSON file which contains additional set of nodes and edges

Benefits

Gives more flexibility.