wid-edit buttons

In #282 (closed) we discussed button styling and improved the default style of the customize buttons (cus-edit) and the eww buttons. It turns out that cus-edit.el uses wid-edit.el and derives its buttons from the wid-edit buttons, e.g., push-button. The faces of the widget buttons are overriden in custom--initialize-widget-variables. Now I wonder if we should simply apply the styling of the box buttons to all the widget buttons for greater coherence and a prettier interface. This way all packages which use widget.el/wid-edit.el directly will get improved looks.

You can check out the widget-example from https://www.gnu.org/software/emacs/manual/html_mono/widget.html#Programming-Example. Compare the looks with and without the following settings:

(setq widget-link-prefix " "
      widget-link-suffix " "
      widget-push-button-prefix " "
      widget-push-button-suffix " ")
(face-spec-reset-face 'widget-button)
(face-spec-reset-face 'widget-button-pressed)
(set-face-attribute 'widget-button nil :inherit 'modus-themes-box-button)
(set-face-attribute 'widget-button-pressed nil :inherit 'modus-themes-box-button-pressed)

Unfortunately, we cannot overwrite the prefixes/suffixes in the theme, but these adjustments could be part of the manual. Here is a screen shoot with and without the adjustments. If we adjust the faces we may want to rename modus-themes-box-button(-pressed) to modus-themes-widget-button(-pressed). This would reduce the confusion regarding button faces and buttons a little bit.