Add rowCount as EnginioQmlModel property

It is really convenient to query row count directly on the model

Task-number: QTBUG-34029
Change-Id: I62bfb6a9c2fe014db778641e85dcb3a406426644
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Remove old data when a query is changed to an empty or invalid object.

The change fixes the issue, that assigning an null object to
EnginioModel::query was not cleaning the internal data cache. It also
fixes exposed assert when an delayed request is sent after a query is
changed to a completely different object and EnginioModel doesn't have
related attached data anymore.

Change-Id: Ifbf4677ee3ed3e91af87c793fbce60977a9932a8
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Remove duplicated byte arrays

The change reduces the binary size by 2kB

Change-Id: I3987a4dc30d57f7116ce6f44e02db1b52197721a
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Fix qdoc link errors

Change-Id: I1a064eb2e562094fc32ca82d2f2d12ed2ff44273
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>