Show app icon in PulseAudio settings

According to the docs, `gtk::Window::set_default_icon_name` should take care of
this, but it evidently isn't. So we'll use the environment variable method
instead.

Fixes #142

podcasts-gtk/src/app.rs

@@ -321,6 +321,9 @@ impl PdApplication {
         bindtextdomain("gnome-podcasts", LOCALEDIR);
         textdomain("gnome-podcasts");
 
+        // Make sure the app icon shows up in PulseAudio settings
+        env::set_var("PULSE_PROP_application.icon_name", APP_ID);
+
         let application = Self::new();
 
         // Weird magic I copy-pasted that sets the Application Name in the Shell.