Use singleton pattern for NewContactDialog

This ensures that only one instance of the new contact dialog can exist
at a time, preventing users from opening multiple dialogs simultaneously
by repeatedly clicking the 'New' button.

v2: Rename 'get_instance()' to 'get_default()' (Erick Pérez Castellanos)

https://bugzilla.gnome.org/show_bug.cgi?id=700889