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
m_Zone is a fully deprecated list of SEG_ZONE items (similar to TRACK), used to fill copper zones in *very old* boards.
it is even unlikely there are still boards that use them.
So it should be removed one day.