72383: When pressing enter in the property sheet, exit edit mode

In addition to fixing the above bug, this CL also improves the testing
infrastructure (for properties it was just calling the underlying
model; it now drives everything through the property sheet UI instead;
this was necessary in order to add a regression test for the above bug
fix.)

Change-Id: Ibb017c1636eee4b329b32a2efef63ce356df11a1
Merge "72383: When pressing enter in the property sheet, exit edit mode" into idea133