In line 5 and 6, Message have two spaces, so I change them to a space.

They just need to register to the PLUGIN_USAGE_DATA event, and then to add
either a simple string, or an array of key / value. For example:

    function register(Doku_Event_Handler $controller) {
      $controller->register_hook('PLUGIN_USAGE_DATA', 'AFTER', $this, 'usage_data');
    }

    function usage_data(&$event){
      $event->data['my_plugin_name'] = 'my usage data';

      //or: $event->data['my_plugin_name'] = array ('k1' => 'v1', 'k2' => 'v2');
    }

- replace PHP4 style class constructor function names (based on
  class name) with php 5 __construct()
Also remove some '&' reference operators used with objects
And add some object type hints

The viewing options should only be visible on the first tab - which is
the installed Plugins tab. (There seems to be no more direct way to
address this tab.)

Since it is a JS function only, the form will be generated by it too.

Allows tho show/hide extensions that are: enabled, disabled, updatable