Speedup net list dialog initialization
For a current design project, I'm in a terrible need of an improved "List Nets" dialog.
As a preparation and general improvement, this one speeds up dialog initialization (when it builds the list and calculates the net lengths). For a larger design with about 40K CN_ITEMs and 2K nets I get a speed up from 2200 ms to 120 ms, where 100 ms are somewhere lost in building the wxWidget list view/data.
In future changes to the net list dialog, I'm probably going to have to re-build the list more often and it would create a significant performance hog.