Allow Mission Editor to change Map but keep all units/triggers/conditions/waypoints (even the ships when it doesn't make sense)

[TEMPEST.114]

This is the only easy way to duplicate triggers/trigger actions and complex flag tree events between 'missions'.

It can flag units that no longer make sense i.e. a Carrier group in the middle of a dessert map, but it would significantly allow the reuse of complex triggers, flags, comm trees and units with all their complex behaviours, to be reused between different missions.