FileUndoManager: use AskUserActionHandler for deletion confirmation

• It's async
• It's an interface in KIOCore, which means one less hurdle for moving FileUndoManager to KIOGui (https://phabricator.kde.org/T11821)

Technically this would break expectations of any code that reimplements UiInterface::confirmDeletion(), no longer called, but nobody was doing that. I wrote UiInterface ages ago so Dolphin could show error messages inline, but confirmDeletion only got there for the unittest.