[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.
Merge pull request #54 from antseppa/profiledeleted

[libsocialcache] Added profileDeleted signal to SyncHelper