AllegroNetlist: Cadence Allegro PCB Designer netlist exporter

1. Added netlist_exporter_allegro.cpp/.h (~1000 LOC) files for the netlist exporter based on David Schneider's original python source file(~200 LOC), and the output of the exporter is Telesis format compatible;
2. Added Allegro notebook page between OrcadPCB2 and CadStar in the "Export Netlist" dialog, and a series of auxilliary variabls, constants and functions were also added to support the new feature.

By David Schneider and Youbao Zhang