proxy: replace global-id with bound-id

+ use the pw_proxy API to find the bound id instead
of relying on WpGlobal

This has the advantage that it works also for exported
objects and for objects that have been created by calling
into a remote factory (such as the link-factory), so we can
now know the global id of all proxies, not only the ones
that have been created by the registry.
2 jobs for api-improvements in 2 minutes and 14 seconds
Status Job ID Name Coverage
  Test
passed #1602212
build

00:02:14

failed #1592883
build

00:02:09