Reuse the same Add Contact window for all incoming requests

[valerio]

If more than one contact adds you when you are not logged in, or if you simply don't close the existing windows before more get opened, KMess will open a different "You've been added by someone" window.

Instead, the window should:

1. Have a better name :P
2. Be a singleton
3. Have a tab bar
4. A new tab should appear when another adding request comes from KMess
5. Close all tabs if you click the window close button.

[valerio]

Another  really nice idea for that dialog, is to add a link to open the Live Spaces profile of the contacts who have added you.

This would easen discovering who they are :)

[valerio]

Remind to add the SOAP request to retrieve the person's friendly name, so that we'll be able to show it here.

 This topic is the reference.