The type hints allow mypy to check the gui_hook calls, revealing a
bunch of places that are broken as they expect no arguments like the
legacy hooks.
To make mypy happy about PyQt's signal.connect(func), a qconnect()
helper has been added.
- No need for the checkbox, as an unchecked box is equal to an empty
`QLineEdit`.
- The value was saved to the profile but not loaded.
- And the real pièce de résistance: I've figured out how to "Promote"
the `QLineEdit` to a `TagEdit`.
Hope you like it! :)
