* 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

Still missing a way to get the default endpoints, but this
is going to change and use the metadata API

... and make WpIterator Lua-friendly so that it can be used
with the generic for loop

Fixes #22

Fixes #20

Use the same set of warning flags that pipewire uses

See #20

a module that allows loading and executing Lua
scripts from a configured directory

... 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.