Make the warning text for deletion operations emphasize its permanency and irreversibility

Test Plan:
Delete a single file:

Delete multiple files:

Empty the trash:

Reviewers: Frameworks, Dolphin, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: broulik

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12420