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.

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'))