helpers/killer: Modernize UI

This overhauls the kill prompt UI to make it more modern and more
easily to grasp.

* Reduce the amount of text and redundancy.
  Give the window a title and drop the "app is not responding" heading.
  Try to remove the application name from the displayed window title.
  Also use the bold emphasis used in other places like deleting files.

* Use the application icon if available with a warning overlay.
  This makes it easier to grasp which application it's talking about.

* Move technical information (like PID and hostname) into an
  expandable "Details" section.

KGuiAddons has been implicitly pulled in by KConfigWidgets already.