Fix QML examples on android

Current android support does not allow to use QtWidgets and QtQuick in
one process, therefore our backend helper had to be rewritten in QML.

Change-Id: I62a1e49be3f92b1aab748221edf434862eec4e09
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Improve EnginioClient::sessionAuthenticated docs

Change-Id: I50e4e659e10d7d2dfced46e6508bbdf5e7f96a3e
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>