Fix HTTP proxy settings

Summary:
After SlaveBase::config() was replaced with SlaveBase::mapConfig()
the reading of "ProxyUrls" value had beed broken as QVariant with
type QByteArray can't be directly converted to QStringList.
It should be converted to QString and splitted.

Reviewers: #frameworks, meven

Reviewed By: meven

Subscribers: meven, kde-frameworks-devel

Tags: #frameworks

BUG: 414346
FIXED-IN: 5.65

Differential Revision: https://phabricator.kde.org/D25432