Add bindings for `pw_loop`.

This adds a Loop struct for creating and managing a pw_loop struct, so that instead of blocking on a mainloop, a loop can be iterated manually (for example, this allows nesting it inside a GMainContext when working with glib).

It also adds a LoopRef struct that is a transparent wrapper around pw_loop, and can be obtained from both a Loop and MainLoop.

The methods previously on the loop trait are moved there, and the loop trait is now only responsible for obtaining such a LoopRef from any loop.

I also added the last missing loop source, IdleSource to call a callback whenever the loop is idle.