Use an event to signal that IM settings are available

Rather than enumerating available classes, define an event that sets a
flag indicating that there's an IM plugin available. Implemented in
implugin.php, so all IM plugins that use that class should
work. Others can hook the event, too.

EVENTS.txt

@@ -1422,3 +1422,6 @@ StartUpgrade: when starting a site upgrade
 
 EndUpgrade: when ending a site upgrade; good place to do your own upgrades
 
+HaveIMPlugin: is there an IM plugin loaded?
+- &$haveIMPlugin: set me to true if you're loaded!
+

lib/implugin.php

@@ -620,6 +620,11 @@ abstract class ImPlugin extends Plugin
         return true;
     }
 
+    function onHaveImPlugin(&$haveImPlugin) {
+        $haveImPlugin = true; // set flag true (we're loaded, after all!)
+        return false; // stop looking
+    }
+
     function initialize()
     {
         if( ! common_config('queue', 'enabled'))

lib/settingsnav.php

@@ -121,7 +121,11 @@ class SettingsNav extends Menu
 
             Event::handle('EndAccountSettingsNav', array(&$this->action));
 
-            if (class_exists('XmppPlugin') || class_exists('IrcPlugin') || class_exists('MsnPlugin')) {
+            $haveImPlugin = false;
+
+            Event::handle('HaveImPlugin', array(&$haveImPlugin));
+
+            if ($haveImPlugin) {
                 $this->action->menuItem(common_local_url('imsettings'),
                                         // TRANS: Menu item in settings navigation panel.
                                         _m('MENU','IM'),