Retitle button "Close" to "Close file" when a file has been removed on disk

Summary: CCBUG: 406305

Reviewers: #ktexteditor, ngraham, cullmann

Reviewed By: #ktexteditor, ngraham, cullmann

Subscribers: cullmann, kossebau, kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D20542