In practice we always create a remote and connect to pipewire.
Any other scenario is invalid, therefore, it is not justified
to be confused with so many classes for such small functionality.
This simplifies a lot the modules code.

Also, this commit exposes the pw_core and pw_remote objects
out of WpCore. This is in practice useful when dealing with low-level
pw and spa factories, which are used in the monitors. Let's not
add API wrappers for everything... Bindings will never use this
functionality anyway, since it depends on low level pipewire C API.

In case the proxy was created with wp_proxy_new_wrap(), the event
listener was not attached on the pw_proxy

* add proxy sync method
* add wrapers for enum/set/subscribe_params
* move the info structure handling to the subclasses
* expose info->props as WpProperties

After discussing things at the AGL May 2019 F2F meeting
and reflecting on the initial design of WirePlumber,
it became clear that it needed a fresh start.

This makes it more intuitive to get other attached interfaces
from the core

We are going to use this in other object types as well

* make the registries available as dynamic interfaces of the core
* improve a bit the C API of WpObject & WpInterfaceImpl