macOS: Closing last terminal window leaves the window title in the menubar

# How to reproduce

- Open Kitty on macOS, note the title of the terminal appears in the menubar.
- Close the terminal (Cmd+W), but do not exit the app

# Expected behavior

The closed terminal's title should be removed from the menu bar

# Actual behavior

The closed terminal's title remains in the menu bar.

# Environment

macOS 10.14, Kitty 0.16.0. Reproduces on empty config.