ConversationTree is now a plugin (not oldschool setting)

Conversation trees works pretty bad with the current layout, javascript
etc. So it's best if we separate it and work on it as a side-project. The
oldschool settings are currently being deprecated (or broken out like this).

I'll wait with removing User preferences for oldschool conversation tree,
since that might be reusable data. But I guess it will go in the near future.

plugins/ConversationTree/ConversationTreePlugin.php

@@ -0,0 +1,46 @@
+<?php
+/*
+ * GNU Social - a federating social network
+ * Copyright (C) 2014, Free Software Foundation, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+if (!defined('GNUSOCIAL')) { exit(1); }
+
+/**
+ * @package     UI
+ * @maintainer  Mikael Nordfeldth <mmn@hethane.se>
+ */
+class ConversationTreePlugin extends Plugin
+{
+    public function onStartShowConversation(Action $action, Conversation $conv, Profile $scoped=null) {
+        $nl = new ConversationTree($conv->getNotices(), $action);
+        $cnt = $nl->show();
+        return false;
+    }
+
+    public function onPluginVersion(array &$versions)
+    {
+        $versions[] = array('name' => 'ConversationTree',
+                            'version' => GNUSOCIAL_VERSION,
+                            'author' => 'Mikael Nordfeldth',
+                            'homepage' => 'http://gnu.io/',
+                            'rawdescription' =>
+                            // TRANS: Plugin description.
+                            _m('Enables conversation tree view.'));
+
+        return true;
+    }
+}