[UX] Use "Configure" icons instead of the full word

Now that we have icons in the admin menu, it strikes me that we could use the "Configure" icon instead of a button that spells out the full word. For example, on the Manage Display pages, a view such as this (admittedly only this many fields are likely to have a "Configure" option if you are using field_formatter_settings but it makes the point):

With a little modification (CSS only) we could have something more like this:

If people agree I can make a PR for Seven that would do this.