change plugin interface for menu links

Currently plugins are supposed to set the public property $cmPluginMenus to expose menu items for the plugin. Since PHP class properties can only be declared using values that are evaluated at compile time the following will not work:

// Expose Menu Items
public $cmPluginMenus = array(
"coperson" => array(_txt('pl.dirviewer.viewmenu') =>
array('controller' => "dir_viewers",
'action' => "view"))
);

Continuing to just have the interface inspect $cmPluginMenus would require putting any non-static description of the menus into a __constructor() method.

Rather than using a __constructor() method, we will change the interface so that the Registry core code base calls the method

public function cmPluginMenus()

{ $menus = array() // Code to set menu content here. return $menus; }

to obtain the menu information from the plugin.