logger: Downgrade some warning messages

A warning implies something went wrong and (possibly) the user should do
something about it. However, warnings are not always used this way.

For example:

* When a fallback value for a `${..}` reference is used, this shouldn't
  produce a warning (or notice) since using fallbacks is not something
  bad.
* pulse telling you that it uses the default sink because no sink was
  specified also does not warrant a warning (even notice may be too
  high).
* Whenever polybar shuts down it produces a "Termination signal
  received..." warning. Since there isn't a more proper way to shut down
  polybar, it should not produce a warning. Same argument for a
  `screenchange-reload`

src/components/controller.cpp

@@ -173,7 +173,7 @@ bool controller::run(bool writeback, string snapshot_dst) {
     m_event_thread.join();
   }
 
-  m_log.warn("Termination signal received, shutting down...");
+  m_log.notice("Termination signal received, shutting down...");
 
   return !g_reload;
 }