Give the user the option of cancelling a file open if there are
segment zones; otherwise they're converted to polygon fills.
Fixes: lp:1823087
* https://bugs.launchpad.net/kicad/+bug/1823087
SEGZONEs are no longer created by pcbnew but may exist in older boards.
This allows deleting the old fills in a manner similar to the deletion
in Legacy canvas.
Fixes: lp:1794571
* https://bugs.launchpad.net/kicad/+bug/1794571
Jeff Young
Seth Hillbrand
Tomasz Włostowski