Opal Accounts: Enable/disable actions when accounts are added/removed.

Ekiga Call Out and Ekiga.net account related actions are now added and
removed following such an account exist or not.
Ekiga App: Regenerate application menu actions when required.

If an Ekiga Call Out account has been added, then recharge, history,
balance... actions become available. If it is removed, then a specific
Action allowing to create a new account appears.

The same behavior is also implemented for Ekiga.net accounts.