... and improve its output

This patch allows configuring an "extension" string that filters
files not only by extension, but also by a subdirectory of the
standard configuration path(s).

The "extension" can now have the following format:
  "subdirectory/path/extension"
which is split on the last '/' and looks for files that end with
".extension" in "WIREPLUMBER_CONFIG_DIR/subdirectory/path/"

If no subdirectory is specified, the code behaves just like it did
before.

All loaded scripts can now be run in a protected environment
so that they cannot do any harm to the host or to each other

This is no longer useful, we are going to use WpSpaPod directly
in combination with the param caching for WpPwObjectMixin

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.

Similar to how pipewire interfaces are versioned.
Keeps the struct extensible without breaking ABI.

This is to mark private functions that are exposed in public headers.
These functions will not be exported from the library and will
generate a warning when client code is trying to use them.

There is no good reason to keep them private

A callback is required, NULL is not accepted by GCClosure

Disable m-session-settings for now, as it needs further work

Now we have a deeper hierarchy, so requesting the features on WP_TYPE_PROXY
wouldn't properly request the features on WP_TYPE_NODE

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.