Change title of the message box to something which conveys message

Summary: The warning message box which appears on applying shift + delete (permanent deletion) combination had title "Warning", which didn't conveyed the actual message to the user.

Test Plan: Shift + delete any file or folder or a combination of multiple files/folders.

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17378