1) Copy label fields when converting.
2) Don't assume netclass directive should be net name -- this is very
unlikely.
3) Don't show font controls in Netclass Directive Properties dialog --
there's no basic text associated witha Directive Label.
Also fixes some plot bugs with arcs.
Also moves polygonization of arcs (when required) in plotting code
from 5 degrees to calculated based on ARC_HIGH_DEF.
Fixes https://gitlab.com/kicad/code/kicad/issues/5017
Jeff Young