[Graph representation] Add new nodes and edges via GUI

Feature description

Allow the user to add new nodes and edges to the graph by clicking on the canvas.

Use cases

A) A user clicks the right mouse button -> he chooses the option 'add new node' -> a new node is created at the position of the mouse click.

B) A user clicks on an existing node with the right mouse button -> he selects 'add new edge' -> an edge is drawn starting from the selected node which the user can connect by dragging with another node

Benefits

Add new nodes and edges via the GUI.