[Shell Handler] Port from own QQmlEngine to using QmlObjectSharedEngine

Authored by broulik on Feb 26 2018, 3:00 PM.

Description

[Shell Handler] Port from own QQmlEngine to using QmlObjectSharedEngine

I noticed in GammaRay there were two QQmlEngine instances in a Plasma session and found this.

Differential Revision: https://phabricator.kde.org/D10774