Some unexpected behavior in @add implementation

[egregors]

@add wrapper eval internal custom function for adding internal mode command handlers.

Next, @add should add mode commands, such a: foo_on, foo_off, foo (for the mode called "foo').

And next, if the default state of mode is False internal mode command handlers will be removed.

BUT! For some reason, if _remove_mode_handlers was not be called, the mode commands will not be work.

What a mess!