Improve recipient management in the BE module

Add the possibility to register multiple source of recipients
and add a button enabling the User to pick one in the GUI.

Snippet:

# Register a new list manager for demo purposes.
Tx_Messenger_ListManager_Registry::getInstance()->add(

    # Corresponds to a class name.
    'Tx_Messenger_ListManager_DemoListManager',

    # A string or label describing the recipients
    'LLL:EXT:messenger/Resources/Private/Language/
     locallang.xlf:demo_list_of_recipients'
);

fixes #10