Fix r99654 change -- rename :on/off-image widget props to :on/off-glyph.

Fix r99654 change -- rename :on/off-image widget props to :on/off-glyph.

* wid-edit.el (visibility): Replace :on-image and :off-image
widget properties with :on-glyph and :off-glyph, for consistency
with the `visibility' widget.
(widget-toggle-value-create, widget-visibility-value-create):
Merge into a single function `widget-toggle-value-create'.

* cus-edit.el (custom-variable-value-create, custom-visibility)
(custom-face-edit-value-create, custom-face-value-create): Replace
:on-image and :off-image widget properties with :on-glyph and
:off-glyph, for consistency with the `visibility' widget.