[applets/kickoff] Add a visible configure button

This allows the new Kickoff's configuration window to be opened without
having to right-click, increasing usability for touch and for people who
aren't in the habit of right-clicking everything.

Many other applets have visible configure buttons now. Kickoff never did
in the past, but this redesign gives us a perfect place to put it.


(cherry picked from commit f2c11f14)