Allow to close a document whose file was deleted on disk

Summary:
There is no confirmation window about data lose.

Screenshot:
{F6766617}

FEATURE: 406305
FIXED-IN: 19.08

Test Plan: Manually tested

Reviewers: cullmann, #kate, #frameworks, dhaumann

Reviewed By: cullmann, #kate, dhaumann

Subscribers: dhaumann, ngraham, kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D20467