In practice we always create a remote and connect to pipewire.
Any other scenario is invalid, therefore, it is not justified
to be confused with so many classes for such small functionality.
This simplifies a lot the modules code.

Also, this commit exposes the pw_core and pw_remote objects
out of WpCore. This is in practice useful when dealing with low-level
pw and spa factories, which are used in the monitors. Let's not
add API wrappers for everything... Bindings will never use this
functionality anyway, since it depends on low level pipewire C API.

Since the alsa endpoints class was renamed...

In case the proxy was created with wp_proxy_new_wrap(), the event
listener was not attached on the pw_proxy

* move the watchdogs in the fixture setup
* use a new GMainContext on each fixture to avoid test dependencies
* test wp_proxy_sync

* add proxy sync method
* add wrapers for enum/set/subscribe_params
* move the info structure handling to the subclasses
* expose info->props as WpProperties

This also adds a "test server" helper, which is used to
start a local pipewire server in the same process, for testing
functionality that requires a server.

When two clients have the same role, the current policy should
favor the newest client.

Error handling and fixes

See merge request gkiagia/wireplumber!21