Improve behaviour of the StartupFeedback

This is not strictly a bug report, but rather a documentation of the various usability improvements the StartupFeedback should get:

• The top and bottom bar of the shell should get opaque already when the StartupFeedback is open, not only when the app has started.
• If the StartupFeedback gets killed by the user, the app the loading of which it indicates should be killed as well
• The StartupFeedback should also be opened if an app starts another app (not only if the shell does it), see !66 (merged)
• The StartupFeedback should not close when it's inactive (plasma-nano!4 (closed)):
  • if the user opens the quick settings while an app is starting, the StartupFeedback is gone.
  • the user of the API in plasma-nano (plasma-phone-components) should handle what happens on activeChanged. We don't need it since we close the feedback using KStartupFeedback with !66 (merged).
• The StartupFeedback should not have a timeout. (plasma-nano!3 (merged)) Some Maui apps take longer than it to start, leading to the feedback window disappearing too early and the app starting nevertheless. If the StartupFeedback really got stuck for whatever reason, the user can still manually close, minimize it etc.