conferences: better handle conf state changes

Never emit callStatusChanged in togglePause and wait for
daemon's signal to do so. Handle ACTIVE_DETACHED and
ACTIVE_ATTACHED in call::to_status().

Change-Id: Ic6d709d37be053e82949b71f628e08631ce135d4
Gitlab: #389
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>