This is very easy to reproduce when the pipewire-alsa integration
is installed and you do 'arecord -l'; the alsa plugin connects and
disconnects again before the proxy is ready.

In this case we have to skip remote-global-added and we also have
to be careful with the references: the global-removed callback is
called earlier, so the core's reference to the proxy is gone and
the GTask is the only thing holding a reference to the proxy.
When we unref the GTask, the proxy is also unrefed, so we have
to keep an additional reference in order to avoid crashing
when accessing the hash table below.

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.