Make notifications execute the "default" action on click.
This uses the feature introduced here  on KNotifications, so default
actions don't appear as a regular action button. Instead, they are
activated by clicking on the notification itself.
If the default action is not set, it uses the previous behavior of close
This changes both the notification popup and the notifications inside the
Test Plan: Manual testing.
Reviewers: Plasma, colomar, mart
Reviewed By: mart
Subscribers: mart, broulik, apol, mck182, plasma-devel
Differential Revision: https://phabricator.kde.org/D4215