engine/scene: preserve fixture order when saving on XML
engine/scene: preserve fixture order when saving on XML

This might be a bit less efficient than before, but it is more consistent in terms of user experience,
especially when empty fixtures are left in a Scene

kripton/qlcplus,sbenejam/qlcplus,mcallegari/qlcplus,plugz/qlcplus,kripton/qlcplus,plugz/qlcplus,mcallegari/qlcplus,kripton/qlcplus,sbenejam/qlcplus,kripton/qlcplus,plugz/qlcplus,kripton/qlcplus,plugz/qlcplus,mcallegari/qlcplus,kripton/qlcplus,sbenejam/qlcplus,plugz/qlcplus,plugz/qlcplus,sbenejam/qlcplus,sbenejam/qlcplus,plugz/qlcplus,mcallegari/qlcplus,sbenejam/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,kripton/qlcplus,sbenejam/qlcplus