Avoid display bugs after editable-field widgets

Avoid display bugs after editable-field widgets

* lisp/wid-edit.el (widget-specify-button): Don't add an
invisible space before the button to avoid bugs that surface
with the widget-field face.  The Widget library is not
the one that puts a :box attribute for the button face, and we
can't control what library users put after editable-field
widgets.

(widget-specify-field): Rather, add the invisible space here,
after the newline.  That way, we protect whatever comes after
the editable-field widget.  (Bug#51550)

(cherry picked from commit 86dd455e9c7402737e2c0c45190fabbf1d3ee8c1)