Improve detecting D&D between two screen showing the same URL

Summary:
If screen 1 & 2 was showing file://home/user/foo and file://home/user/foo/bar
was moved from one screen to another, it was not detected as a drop
between shared views and the Copy/Move/Link menu was shown.

Reviewers: #plasma, hein, mwolff

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10729