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
SELECTION now holds EDA_ITEMs not BOARD_ITEMs so various places had to
change to casting the selected items to BOARD_ITEMs.
Fixed compilation warnings on clang (Tom)
When creating a copy of CONTEXT_MENU, always a CONTEXT_MENU instance was
constructed, whereas an inherited type should be used. Solved with
CONTEXT_MENU::create() that has to be overridden in inheriting classes.
Event & update handlers are now virtual functions, instead of setting
the handlers with Set{Event,Update}Handler().
Jeff Young
Seth Hillbrand
Tomasz Włostowski
Jon Evans
Maciej Suminski
Miguel Angel Ajo