loop: pass spa_dict to *_loop_new

Make the thread_loop alloc its own loop by default to simplify
some core. Add extra new_full method to pass a custom pw_loop.
Make other loop implementations ready to support custom loops
if we want that later.

src/gst/gstpipewiresrc.h

@@ -71,8 +71,7 @@ struct _GstPipeWireSrc {
   GstClockTime min_latency;
   GstClockTime max_latency;
 
-  struct pw_loop *loop;
-  struct pw_thread_loop *main_loop;
+  struct pw_thread_loop *loop;
 
   struct pw_context *context;
   struct pw_core *core;