FEATURE: support for emoji sets

Added following emoji sets
  - Apple/International
  - Emoji One (default)
  - Android/Google
  - Twitter

FIX: translations from plugins weren't properly merged with default translations
FEATURE: new 'site_setting_changed' event

lib/plugin/instance.rb

@@ -95,6 +95,11 @@ class Plugin::Instance
     end
   end
 
+  def listen_for(event_name)
+    return unless self.respond_to?(event_name)
+    DiscourseEvent.on(event_name, &self.method(event_name))
+  end
+
   def register_css(style)
     @styles ||= []
     @styles << style