Issue #3027653 by clayfreeman, tim.plunkett, Pavel Ruban, raman.b,...

Issue #3027653 by clayfreeman, tim.plunkett, Pavel Ruban, raman.b, ankithashetty, cboyden, larowlan, mpotter, phenaproxima: Allow block and layout plugins to determine if they are being previewed

(cherry picked from commit 1c4a6d6c)