avoid adding entry in RecentModel after an incoming ContactRequest

- when we get an incoming ContactRequest we build a Person from the
vcard nested in the payload. We next set this Person object to a
ContactMethod which trigger the signal contactChanged. When this
signal is emitted, RecentModel adds an entry.

- this patch adds a check before adding an entry in the RecentModel,
if the current changed is for an actual contact or a ContactRequest.
In last case, it doesn't create a new entry.

Change-Id: I82c03ade965e65f5e32cdc7ecf81b527c4c3aeee
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>