Institutes property checks for non-event context property settings.

Institutes property checks for non-event context property settings.

This fixes issue 1734 in addition to any potential incorrect settings
of properties in the layout block.  While this entails a property check
for all property settings in the ly/ folder, the time this takes is
negligible.
(cherry picked from commit a811a3c91c05f33474c1d447bedaa1e089522532)