This commit improves the performance of QContactDetail by avoiding
use of QMap<int, QVariant> to store data, as the memory footprint
of this storage container is prohibitively large.

Instead, all details' fields are now backed by specific data members
and the QMap is only used in fallback or extension cases.

According to the tests/benchmarks/contacts/detailsbenchmark results,
this commit reduces memory usage by about 35% and reduces runtime
by about 35% also.

Change-Id: Ie73d124563ebf4b46d35810116110322af2208c4
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>