Add ability to put a header at the top of a toolGroup pop-up

Needed for
https://trello.com/c/xCo6uLAZ/9-5-references-dropdown-from-toolbar-styling

Change-Id: Id54100c55b05bdae5858ca72e4fd4418ceff0454
Merge "Add ability to put a header at the top of a toolGroup pop-up"