Fix issue in Qt 5.7

Summary:
qml parser doesn't like having namespaced object types in the declaration
of a property. Use QtObject instead.

BUG: 380435

Test Plan:
Tests pass, applied it on my neon machine (sudo vim /usr...) and
it seemed to work

Reviewers: #kirigami, jriddell, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D6065