Right now, OO.ui.PopupWidget allows for a header, but the option takes header text only, and inserts it as a label. Also, if a header exists, we automatically get an "x" close button - that's not always necessarily needed, and the fact of having a header shouldn't be the thing that decides that.
- Allow for an icon in the header, as follows from most other interfaces we have, that if exists, is placed before the header label
- Make the 'x' close button a config option. Ideally, it would appear regardless of having a header, but at the very least give the option so it will not always appear with a header either.