Remove implicit use of QtQml in Qt 5.7

Summary:
The Plasma 5.8 branch should work with Qt 5.6.
This means we don't have the Connections enabled property available.

This moves the enabling logic into the handler.

BUG: 380240

Test Plan:
Ran Plasma 5.7
Grouped some things
Created a popup
Moved mouse to another task
Seemed normal

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6007