Expose JavascriptCanPaste attribute

Expose an attribute to enable the execCommand("paste") functionality,
which is by default disabled due to security concerns.

[ChangeLog][General] Added JavaScriptCanPaste attribute that
enables JavaScript 'execCommand("paste")'.

Task-number: QTBUG-64056
Change-Id: I1a414021e020473dd15946fff91fb103d871e961
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>