This is for the out-of-box experience for novice users. It is presumed
that folks with larger more complicated boards will turn it off.
Fixes https://gitlab.com/kicad/code/kicad/issues/6413
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