utils InstanceRegistry: return instances sorted from old to new.
InstanceRegistry: ensure keys are numbers.
InstanceRegistry: new function getLastInstance.
Circuit Graph Plot: the overlay rect should be at the bottom
so that nodes are selectable and tooltips appear on mouse over.
Graph widget: implemented buttons 'Export Adjacency Matrix" and "Open plot"

The adjacency matrix is created for visible nodes and edges.
A new Circuit Graph Plot widget instance is opened.