Don't close popup when spring-load dragging into it.

Summary:
I'm not sure how this regression came about. It might be due to mouse
handling changes on the Qt 5.9 branch. I'd appreciate it if someone
tests spring loading with this patch and older Qts.

Reviewers: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5310