[libcommhistory] Don't resolve contacts by default for CallModel and ConversationModel

Contact resolution is expensive, not always available, and infrequently
used. It should be opt-in for all models where it's not necessary for
functionality.

This is an API change. Models interested in contacts should opt-in using
setResolveContacts.
Remove redundant calls to enableContactChanges(false)
[libcommhistory] Add resolveContacts property for declarative models
[libcommhistory] Add contactIds and contactNames roles for EventModel

Exposing the QPair of contacts to QML serves no purpose; it can't be
used there.
Merge pull request #70 from special/contact-not-default

[libcommhistory] Only resolve contacts for event models when explicitly requested