In release 0.7.0, Cancel in the Preferences dialog (opened via Options
in the File tab) did nothing. This was previously addressed on main by
#757 (#752).
This change adds an egui_kittest regression test verifying that clicking
Options from the File tab opens the Preferences dialog and clicking
Cancel closes it while keeping Backstage open. In addition, the About
dialog's Cancel button now properly closes the dialog instead of being
ignored.
Closes#782.