Use the much faster urls() method from QMimeData

Summary:
When requesting a list of text/uri-list, use the much faster QMimeData
urls() method.
The unittests pass (the desktop: protocol) and
CCBUG: 342056
is half solved. The paste speed is as fast as drag&drop in local files
and with fish: files.
The lock in X11 plasma or kwin still needs another patch.

Test Plan: Select 2000 files in one folder, cut and paste them in another disk.

Reviewers: #frameworks, dfaure

Reviewed By: dfaure

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D10155