Redesign Autostart KCM

This is a redesign of the Autostart KCM according to today's standards.

I made a few choices that might be controversial:
- It is not possible to disable applications any more without removing them from the list
- It is not possible any more to control whether something is only autostarted in Plasma
- Scripts are now always symlinked, there is no option to copy them any more
- It is not possible any more to add scripts to ~/.config/plasma-workspace/env. Existing scripts in this directory still show up, in the same category as the other autostart scripts