Allow viewing all options in tag field on focus#427

From a content editor’s perspective, it would be helpful to show the list of options in a tag field when it’s focused. Currently, the user has to start typing for a subset of the options to show. What I’m proposing is an opt-in option to change this so that all options show when the field is focused. Especiall for cases where the user can only select from the supplied options, this behavior would offer a much improved UX.