Set the state of the item to unchecked. Note that this is only reflected in the context menu if the function AddPopup() is used.
inst Uncheck index|key=value
The index of the item to return (as returned by the CMITEM message).
A key/value pair. The key must be one of the keys supported by AddItem(). The value should be a value previously passed to AddItem().
An empty string on success and an error code on failure.