Replace KLocalSocket with QLocalSocket

Summary:
As described in T12010.

AFAIU the QTcpSocket path was only used for Windows, but since QLocalSocket supports Windows we can just use that.

This is entirely untested on Windows though. @vonreth @cullmann can you give this a try?

Test Plan:
Did some SFTP stuff in Dolphin.
No new tests fail

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: cullmann, vonreth, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26034