put user agent in one place

src/api.vala

@@ -142,7 +142,7 @@ public class Subsonic : Object {
         this.parameters = @"u=$(Uri.escape_string(username))&t=$(Uri.escape_string(token))&s=$(Uri.escape_string(salt))&v=1.16.1&c=eu.callcc.audrey";
 
         this.session = new Soup.Session ();
-        this.session.user_agent = "audrey/linux";
+        this.session.user_agent = Audrey.Const.user_agent;
 
         this.artist_list = new ListStore (typeof (Artist));
         this.album_list = new ListStore (typeof (Album));

src/globalconf.vala

@@ -0,0 +1,3 @@
+namespace Audrey.Const {
+  public const string user_agent = "audrey/linux";
+}

src/meson.build

@@ -1,6 +1,7 @@
 audrey_sources = [
   'api.vala',
   'application.vala',
+  'globalconf.vala',
   'main.vala',
   'mpris.vala',
   'playbin.vala',

src/playbin.vala

@@ -41,7 +41,7 @@ class Playbin : Object {
     private bool next_gapless;
 
     private void source_setup (Gst.Element playbin, dynamic Gst.Element source) {
-        source.user_agent = "audrey/linux";
+        source.user_agent = Audrey.Const.user_agent;
     }
 
     // ASSUMPTION: about-to-finish will be signalled exactly once per track