Palette: add header

Palette will have a header if the user pass primaryText to the
constructor.  If the user sets content, there will be a separator
between the header and the content.

API change: to set the content, use palette.setContent(content) .
Previously was done using getContainer(), which does not exist
anymore.

Also, improve the offset calculation using
element.getBoundingClientRect
<https://developer.mozilla.org/en-US/docs/Web/API/element.getBoundingClientRect>