Copy disabled when cell_selectable=False

[mari-pwbm]

Hello!

I've noticed that setting cell_selectable=False disables all copying of the contents of the table, even when editable is set to true.

As an aside, it seems like cell_selectable=False, editable=True re-enables normal text selection but copying to clipboard is disabled. Ideally, we'd have normal text selection just with the option to copy.

Also, it looks like the column headers also cannot be copied.

[wbrgss]

related: #837

[benedikt-budig]

see also: https://github.com/plotly/dash/issues/1978