glw: If PROP_DEBUG is defined, make trace() print pointer to subscription
prop: Move /subtrack HTTP API to prop_http.c
prop/http: Print subscribers when showing info about a property
prop: Better debug for tracked subscription
navigator: Don't create a link for $page.origin -> model since it doesn't work

This is a bit of a workaround for a yet to be fixed problem with
property unlinking

Fixes #2419