Final round of review comments

- Fix leak and crash when job goes away before being shown
- Some emit and signal fixes
- Always create services watchers
- Update unity job percentage in destructor, fixes the progress lingering around when disabling in settings
  Also tell the world that we're merely proxying a job progress
- Don't hardcode KIO error codes
- Honor "show critical on top of full screen" setting
- Don't show generic "dialog-information" icon, it's pointlessly generic