Allow other clients to preempt the channels we are handling

This is needed if we are handling an incoming text channel and then user tries
to open a chat with the same contact using Empathy. In this case, the Shell
should delegate the channel back to Empathy and just continue observing it as
it does for usual outgoing channels.

Depends on telepathy-glib 0.15.3 as
tp_base_client_set_delegated_channels_callback() has been added in this
version.

https://bugzilla.gnome.org/show_bug.cgi?id=654237

configure.ac

@@ -73,7 +73,7 @@ GIO_MIN_VERSION=2.25.9
 LIBECAL_MIN_VERSION=2.32.0
 LIBEDATASERVER_MIN_VERSION=1.2.0
 LIBEDATASERVERUI_MIN_VERSION=2.91.6
-TELEPATHY_GLIB_MIN_VERSION=0.15.0
+TELEPATHY_GLIB_MIN_VERSION=0.15.3
 TELEPATHY_LOGGER_MIN_VERSION=0.2.4
 POLKIT_MIN_VERSION=0.100
 STARTUP_NOTIFICATION_MIN_VERSION=0.11

js/ui/telepathyClient.js

@@ -91,6 +91,11 @@ Client.prototype = {
         this._tpClient.set_handle_channels_func(
             Lang.bind(this, this._handleChannels));
 
+        // Allow other clients (such as Empathy) to pre-empt our channels if
+        // needed
+        this._tpClient.set_delegated_channels_callback(
+            Lang.bind(this, this._delegatedChannelsCb));
+
         try {
             this._tpClient.register();
         } catch (e) {
@@ -245,6 +250,11 @@ Client.prototype = {
                               requests, user_action_time, context) {
         this._handlingChannels(account, conn, channels);
         context.accept();
+    },
+
+    _delegatedChannelsCb: function(client, channels) {
+        // Nothing to do as we don't make a distinction between observed and
+        // handled channels.
     }
 };