Fix kio-extras compilation with -DQT_NO_CAST_TO_ASCII

Summary:
D27804 broke compilation for me because I have the above flag set ;-)
It showed rather some rather nasty QString -> QByteArray conversions in this code.

Test Plan: Now it builds.

Reviewers: thiago, sitter

Reviewed By: sitter

Subscribers: meven, kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D29149

(cherry picked from commit 02b566331071ca22e41feaac6aa34448983af48f)