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.

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

* Use a direction instead of a string prop id
* Emit default-endpoint-changed again
* Get rid of the useless virtual methods

+ 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

+ use a more correct naming scheme to match the one from Spa
+ remove the default video source, since we now use a different
 session for video

- 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

* core no longer exposes create_remote/local_object
* node, device & link have constructor methods
  to enable the create_remote_object functionality
* added WpImplNode to wrap pw_impl_node and allow creating
  "local" node instances
* added WpSpaDevice to wrap spa_device and allow creating
  "local" device instances
* exporting objects in all cases now happens by requesting
  FEATURE_BOUND from the proxy, eliminating the need for WpExported
* replaced WpMonitor by new, simpler code directly in module-monitor
* the proxy type lookup table in WpProxy is gone, we now
  use a field on the class structure of every WpProxy subclass
  and iterate through all the class structures instead; this is
  more flexible and extensible

These are set by the server on the global properties set

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.