KIO: remove waitForStarted() from the launcher jobs public API

Summary:
This only exists for KRun. I don't want apps to start using that.
We have to be able to remove waitForStarted() in KF6 so that we can do other
async things before starting the process (like resolving URLs, making async dbus
calls for FUSE stuff etc.)

• Rename KRun::KRunPrivate to KRunPrivate so that we can declare it as friend in the job classes
• Move two file-static methods in krun.cpp into the KRunPrivate class so they benefit from this

Test Plan: It builds

Reviewers: davidedmundson, broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28390