Fix QNetworkAccessManager crash at application shutdown

Changed textures to use a common network access manager from
QML engine instead of each of them creating their own.

Change-Id: Ia70a756edc7f3c32679965a8edf28fa64be86dd4
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>