[libcontacts] Handle presence updates independently

qtcontacts-sqlite allows clients to handle presence change notifications
separately from other contact change notifications.
[libcontacts] Optimize fetching of presence updates
[libcontacts] Remove sorting from incidental fetch queries

The order of contact processing in update queries has no relevance.
[libcontacts] Do not sort refetch contact details

Sorting has no effect after the models are initially populated.
[libcontacts] Stop pretending that we support multiple engines

libcontacts will only work correctly with the qtcontacts-sqlite
contact manager engine; remove the use of the NEMO_CONTACT_MANAGER
environment variable from qt4 code.
Merge pull request #60 from matthewvogt/presence-updates

Testing PR 59 in 'next'