🍒 WidgetExplorer: improve header margins

Part of teams/vdg/issues#74.

Make them larger and consistent on the left, right, and bottom, which looks better. And reduce them on top which centers the header text in the space and makes the buttons respect Fitts' Law.

(cherry picked from commit 3172654b)

Co-authored-by: Nate Graham nate@kde.org