header sorting is not work in HTML Column.

[y-kawakami-ccs]

e.g.

columns: [
 { title: `<div>ONE</div><div>TWO</div>` }
],
stripHTML: false,

When doubleclick on header, e.target is div element. it has not "data-x" attribute.

I think that should be get td element in getElement or new method.

I am concerned about the overlap between custom HTML headers and sort markers, but I think this should be handled by the user. Currently, there is still overlap in very small columns.

I am also concerned that HTML cannot be used in Nested Header.

-- sorry, i'm not good at english.