Don't discard customizations in progress when adding comments (Bug#5358)

Don't discard customizations in progress when adding comments (Bug#5358)

* lisp/cus-edit.el (custom-comment-show): Add docstring.  Save the
widget value in the :shown-value property, before redrawing.
(custom-variable-modified-p): New function, to complement the return
values of custom-variable-state.
(custom-variable-state-set): Use it.
(custom-face-value-create): Add children to the custom-face widget
before setting the state, to be able to check for user edits.
(custom-face-state-set): Check for user edits before calling
custom-face-state (bug#5358).

* test/lisp/custom-tests.el
(custom-test-show-comment-preserves-changes): New test.