Add closed files to recent files

This merge request adds closed files to the list of recent files.

Closed files actually are added to the list of recent files most of the time because they are added when switching to their tab. But if one clicks on the close button or kills the tab with a middle click, the document does not get added and it can be difficult to know which document one inadvertently closed.

The first commit fixes the issue. The second one is a suggestion to use the KateMainWindow::slotOpenDocument slot for open file since it exists and is not used elsewhere. This commit can be safely discarded.

Bugzilla ticket: https://bugs.kde.org/show_bug.cgi?id=453691