fixture editor: improve "acts on" update
fixture editor: improve "acts on" update

No need to update the whole mode channel list every time.
It makes the mode editor unusable when there are a lot of channels

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