... 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.

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