Allow abortion of starting another NativeAppJob

Summary:
- offers to cancel an accidental startup of an already running job
- kill all running instances instead of one by one
- fix: do not ask again if "No" was already selected

Test Plan:
- start second instance of an already running job
- select
    * Kill All Instances (No) (kills all running instances)
    * Start Another (Yes) (just starts a new instance)
    * Cancel (closes the dialog and drops the new job)

Reviewers: #kdevelop, mwolff

Reviewed By: #kdevelop, mwolff

Subscribers: kfunk, mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8199