Solved the "Title of document tab/subwindow is not updated after changing document title" bug

Summary: The issue was that when the current document title was changed (by using Document Information dialog for example), its name shown in the files tab remained the same, without being updated. I searched for the method which should update the document name and called it.

Reviewers: Krita

Subscribers: dkazakov, woltherav

Tags: Krita

Differential Revision: https://phabricator.kde.org/D4462