Fix proxy KCM not loading manual proxies correctly.

Summary:
commit 4e2a52c671 prepends a scheme (like socks://), which interfers with
the assumption that after the first ':' comes the port.
But we can use QUrl parsing to extract the port number, instead.

Test Plan: kcmshell5 proxy, setting manual proxy values, OK, kcmshell5 proxy

Reviewers: dmariano, davidedmundson

Reviewed By: davidedmundson

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7142