Issue #2919795 by tim.plunkett, tedbow, dead_arm, webchick, alexpott, xjm: Add...

Issue #2919795 by tim.plunkett, tedbow, dead_arm, webchick, alexpott, xjm: Add visual hints that Layout Builder work is in tempstore and will not be lost, or take effect until saved

(cherry picked from commit ebcf1459)