kcms/ksplash: improve behaviour of delete button

Sam Crawfordrequested to merge
samlkcrawford/plasma-desktop:work/kcm-delete-buttons into master

Some improvements for the delete button of the splashscreen kcm, akin to plasma-workspace!6222 (merged).

  • hide delete button for the "None" option
  • Don't allow deleting the selected splash screen
  • Change tooltip based on whether trying to delete a selected/system-installed/removable splash screen

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