[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.
Unfortunately, this might not be the case in some situations (eg, CalDav sync plugin doesn't use caldav.Calendars-AccId.xml unfortunately, it just uses caldav-AccId.xml).  But for every other type it should work.  Perhaps we should update the caldav plugin to use the "standard" profile name form...