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