* Use a more complete API to introspect SPA types
* Avoid the need for the Tables enumeration; the tables
  are now registered with a string
* Avoid the need for initialization, work directly on spa_types
  and other static data
* Allow working with Object pods that are not Params;
  the PARAMS table was previously hardcoded in the pod implementation
* Add a different dynamic type registration system, closer to
  how spa type works. The only regression is that we can no longer
  register additional custom object fields (custom SPA_PROP_* for example),
  but this feature can be re-added later

Fixes #20

Now the WpPipewireObject interface is directly implemented by the mixin
and there is another interface that users of the mixin must implement
in order for the mixin to work proprely.

A lot of manual stuff that proxy classes had to do before are now
in the mixin. Also most of the data that would normally reside in Private
structures is now in the mixin data structure (stored as qdata on the object).
This is achieving the best amount of code reuse so far.

For impl objects (WpImpl*) there are also default implementations of the
standard pipewire object methods and the INFO & PARAM_* features are
more coherently enabled during the whole lifetime of these objects.

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

This reverts commit 917a019b.

- make it a GObject so that it can emit its own signals
and so that it can be shared between multiple proxies
- share the WpProps instance between endpoints, endpoint-streams
and their underlying nodes
- introduce the concept of the caching mode that redirects _set
to _set_param of the proxy that actually has the props; this allows
shared WpProps to actually set changes on the correct proxy
in a transparent way
- change methods to consume the ref of the pod and reflect that
also on wp_proxy_set_prop()
- refactor the export process on endpoints & endpoint-streams
so that they always get all the required features (info, props, bound)
and make it async so that we can take time to prepare the underlying
node to have FEATURE_PROPS
- update the props & endpoint unit tests, bringing back all the
checks that the endpoint unit test used to have

+ 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

We can now call wp_proxy_request_destroy() on endpoint links and
the WpImplEndpointLink together with the session item that created
it will be cleaned up

- Use similar code for consistency
- Add changed signals everywhere
- Port to the new object-manager API

+ add the useful _find_proxy() method

+ enable the new log writer on the executables
+ enable structured logging in the tests

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)