This is an attempt to unclutter the API of WpProxy and
split functionality into smaller pieces, making it easier
to work with.

In this new class layout, we have the following classes:

- WpObject: base class for everything; handles activating
|           and deactivating "features"
|- WpProxy: base class for anything that wraps a pw_proxy;
 |          handles events from pw_proxy and nothing more
 |- WpGlobalProxy: handles integration with the registry

All the other classes derive from WpGlobalProxy. The reason
for separating WpGlobalProxy from WpProxy, though, is that
classes such as WpImplNode / WpSpaDevice can also derive from
WpProxy now, without interfacing with the registry.

All objects that come with an "info" structure and have properties
and/or params also implement the WpPipewireObject interface. This
provides the API to query properties and get/set params. Essentially,
this is implemented by all classes except WpMetadata (pw_metadata
does not have info)

This interface is implemented on each object separately, using
a private "mixin", which is a set of vfunc implementations and helper
functions (and macros) to facilitate the implementation of this interface.

A notable difference to the old WpProxy is that now features can be
deactivated, so it is possible to enable something and later disable
it again.

This commit disables modules, tests, tools, etc, to avoid growing the
patch more, while ensuring that the project compiles.

they are not useful anymore because we hijack priv->info from the
impl subclass, so that it points to the impl info struct,
and therefore the base implementations work just fine

+ rename FEATURE_CONTROLS to FEATURE_PROPS
+ add accessor for the standard spa_param_info (info->params)
+ hide the low-level params API that nobody uses

Features are flags, therefore we must NEVER use them without a shift,
otherwise bad mistakes happen, like the previous mistake of declaring
WP_SESSION_FEATURE_LINKS as the number after WP_SESSION_FEATURE_ENDPOINTS,
which ended up being (WP_SESSION_FEATURE_ENDPOINTS | WP_PROXY_FEATURE_PW_PROXY)
and it was always becoming available together with the ENDPOINTS feature.

+ add the useful _find_proxy() method

This feature enables the caching of WpEndpointStream objects
inside WpEndpoint, making information about streams readily
accessible to the user of an endpoint

* introduces API to export session items
* introduces small changes in the WpSiEndpoint & WpSiStream
  interfaces to make it nicer to work with
* ports WpImplEndpoint to use PW_TYPE_INTERFACE_Endpoint
  to export. Depends on:
  https://gitlab.freedesktop.org/pipewire/pipewire/-/merge_requests/246
  (was merged after 0.3.2)

heavily based on the WpSession implementation

to avoid name clashing with the upcoming WpEndpoint interface
that is going to be the common interface of Wp{Proxy,Exported}Endpoint

* rework how global objects are stored in the core
* rework how users get notified about global objects
  and proxies of remote global objects

The purpose of this change is to have a class that can manage
objects that are registered in the core or signalled through the
registry. This object can declare interest on certain types
of global objects and only keep & signal those objects that it is
interested in. Additionally, it can prepare proxy features and
asynchronously deliver an 'objects-changed' signal, which is
basically telling us that the list of objects has changed.

This is useful to simplify port proxies management in WpAudioStream.
Now the stream object can declare that it is interested in ports
that have "node.id" == X and the object manager will only maintain
a list of those. Additionally, it will emit the 'objects-changed'
signal when the list of ports is complete, so there is no reason to
do complex operations and core syncs in the WpAudioStream class
in order to figure out when the list of ports is ready.

As a side effect, this also reduces resource management. Now we
don't construct a WpProxy for every global that pipewire reports;
we only construct proxies when there is interest in them!

Another interesting side effect is that we can now register an
object manager at any point in time and get immediately notified
about remote globals that already exist. i.e. when you register
an object manager that is interested in nodes, it will be immediately
notified about all the existing nodes in the graph. This is useful
to avoid race conditions between connecting the signal and objects
beting created in pipewire

+ add a method to retrieve the core
+ change the register function to not require the core as a parameter

This changes the registration point to be the endpoint and not the
session manager, as it seems easier to implement (the implementation
will not need to keep a pointer to the session manager around)

* Make streams a GVariant array, for future-proofness
* Add API for controls (volume, mute, brightness, contrast, etc...)
* Remove API for profiles (it's not well-thought; may be re-added
  in the future)

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.