Move more functionality from KRun::runCommand/runApplication to KProcessRunner

Summary:

• Make that functionality QWidget-independent
• Emit errors as a signal rather than a messagebox. KRun still connects

to the signal and shows messageboxes, so there's no behaviour change for
KRun users.

This is one more step in the direction of OpenUrlJob+KProcessRunner,
both in KIOGui.

Next steps: turning it into a KJob, then renaming it to
e.g. KIO::ProcessLauncherJob

Test Plan:
I'll write a unittest after conversion to KJob.
For now I used the GUI kruntest program.

Reviewers: vkrause, nicolasfella, broulik

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23944