KWindowSystem: deprecate KStartupInfoData::launchedBy, unused

Summary:
KRun bothers with a WId in many places just to end up calling
KStartupInfoData::setLaunchedBy... and nobody uses that value anywhere.
Time to deprecate this and simplify KRun (which I'm rewriting).

Test Plan: Builds. Warns when used in KIO.

Reviewers: zzag, broulik, davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28016