Most native Mac applications show the registered icon in the titlebar when a known document type is opened, and provide a path dropdown menu under that icon. Selecting one of the path components in that menu opens the corresponding folder in the Finder. The icon can also be "grabbed" with a mouse click and then dragged and dropped onto terminal windows and file dialogs (that is probably the more interesting feature).
Qt exposes this feature via `QWidget::setWindowFilePath()`; this change calls the method in `MainWindow::updateCaption()`.