This is more reliable than assuming the link carries the id of its nodes, as there have been cases where a link was created without those
properties set.
Instead, we can just pull them from the state via the port ids of the link.
This extends the `Application` struct to keep more advanced state.
This state is then used to determine the needed information to create
or delete a link between the two connected ports.
A message to create/delete the link is then send to the pipewire thread,
which executed the request.
The `View` sturct was mostly a layer of indirection, and the controller benefitted by absorbing the gtk::Application
subclass parts, so now those two are merged into a new gtk::Application subclass.
