Improvements for Length Notifier

The following discussion from !183 (merged) should be addressed:

1. Instead of storing data in the chat control length_notifier_plugin_data, create a Class that inherits from Gtk.Label() store all information in that class.

2. connect to the buffer from that class, so on the callback you are already inside the Label and have all data available, so you dont have to pass the chat control to the callback and retrieve the data from the chat control

3. Color change should be done via adding a css class and removing a css class, not via override_background_color which is deprecated

4. Migrate to new style plugin config window