properties: use a boxed WpPropertiesItem type when iterating

Needed for JavaScript bindings
7 jobs for properties-item in 58 seconds (queued for 14 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #11847865

00:00:11

passed container_ubuntu #11847866

00:00:11

 
  Build
passed build_on_fedora_no_docs #11847868

00:00:45

passed build_on_fedora_with_docs #11847867

00:00:47

passed build_on_fedora_wpipc #11847869

00:00:42

passed build_on_ubuntu_no_gir #11847871

00:00:44

passed build_on_ubuntu_with_gir #11847870

00:00:46