bindableproperties is the actual project, not a subdirectory with
separate examples. It should use qt_internal_add_example.
Pick-to: 6.6 6.7
Task-number: QTBUG-90820
Task-number: QTBUG-123096
Change-Id: I409d20e035956e6c236f84721b3c6882893cd547
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>