PipeWire headers were recently cleaned up to reduce the number of
included headers. This leads to a number of functions and types not
being included when needed in wireplumber.

This is very easy to reproduce when the pipewire-alsa integration
is installed and you do 'arecord -l'; the alsa plugin connects and
disconnects again before the proxy is ready.

In this case we have to skip remote-global-added and we also have
to be careful with the references: the global-removed callback is
called earlier, so the core's reference to the proxy is gone and
the GTask is the only thing holding a reference to the proxy.
When we unref the GTask, the proxy is also unrefed, so we have
to keep an additional reference in order to avoid crashing
when accessing the hash table below.

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.

signal emission apparently does not work in _finalize()

Fixes some critical warnings from remote-endpoint.c because disconnection
from the destroyed endpoints did not happen

This avoids being able to find the same object on the globals list
from code that is connected to the signal.

This is a cleaner way to interface with the remote pipewire daemon.
The WpRemote base class can be subclassed also for interfacing
with other daemons (hardware-specific managers, etc)

and also implement
* global-added/removed signals
* a foreach function to iterate through globals
* registering modules and factories with the same key

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.