Use KRun::runApplication, add missing KActivities::notifyAccessed calls.

Summary:
Otherwise we:

• ... don't get startup notifications.

BUG:381500

• ... don't contribute data to recent/most used apps.

This is a surprisingly large oversight.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6354