Fix EnginioModel behaivour when an cached object was removd externally.
Fix EnginioModel behaivour on a network error.

We know that a request didn't succeed so the old value is probably
the current one, but the code was missing dataChanged signal that would
inform a view about the revert.
Move attached data to one place.

This will help us to collect all use cases and write a proper container.
Simplify EnginioReply::isFinished.
Never delete QNetworkReply directly

It may happen that setNetworkReply is called as a result of finished
signal, in such situation deleting the reply directly causes undefined
behaviour.
Allow to remove not synced items from EnginioModel.

Removing operation requires an object id as part of query.
It is not a problem, even for not synced objects to get and
use it. The only issue was if the object didn't have id, because
it's create query was not finished. In such cases we postpone the
remove request until creation is confirmed and we have the real
object id.
Merge pull request #186 from nierob/master

EnginioModel improvements.