Port some core Q_PRIVATE_SLOTS to new connect syntax

Summary:
KJob's are quite QObject heavy, we lose a non-trivial amount of time in
signal matching strings. This ports some Q_PRIVATE_SLOTS in FileCopyJob
and the Job base.

Test Plan:
Relevant unit tests
Copied some files

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17260