[qtcontacts-sqlite-extensions] Allow TWCSA implementations to override detail delta detection

This commit adds API to TWCSA::storeRemoteChanges() which allows the
specific implementation to override the detail delta detection
logic of the base-class, as in some cases that logic isn't precise
enough (and the specific implementation has more data about how to
do the mapping correctly).
Merge pull request #122 from chriadam/overridedelta

[qtcontacts-sqlite-extensions] Allow TWCSA implementations to override d...