improve toggling split editor state: in particular, fix https://github.com/sagemathinc/smc/issues/1044
improve toggling split editor state: in particular, fix https://github.com/sagemathinc/smc/issues/1044

tscholl2/smc,sagemathinc/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc