[libsocialcache] Added profileDeleted signal to SyncHelper

SyncHelper notifies when a profile is being synced and UI can then refresh
data after the sync is over. However, that mechanism cannot be used to
refresh data after a profile has been deleted. Added a new signal
for that purpose.
You're right, that seems ot be the case. At the moment it's not likely to cause much trouble as we only provide QML-models for limited set of services and those are the ones we monitor through SyncHelper, but I added a task about unifying it.