Add a setter to TagWidget's parent

[brichet]

A getter has been added to the TagWidget's parent attribute in v4.0.0a28, without setter (which should rely to its parent class).

It look like Typescript does not allow overriding the getter without the setter and vice versa.

At the end, it is not possible to add a new tag to a cell :

Code changes

Add a setter to TagWidget's parent attribute.

Backwards-incompatible changes

None

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