Enable/disable buttons, tool bar and menu items in Custom buffer (Bug#14398)

Enable/disable buttons, tool bar and menu items in Custom buffer (Bug#14398)

* lisp/cus-edit.el (custom-reset-extended-menu): Keymap menu for the
Revert... menu button.
(custom-reset-menu): Keep for backward compatibility, but default to
nil, so we prefer the keymap menu instead.
(custom-reset): Pass the new keymap menu to widget-choose.

(custom-commands): Add an element to each list item, to manage its
enable/disable state.  Add docstring.
(custom-command-buttons): New variable, to hold the buttons that act
on all options in a Custom buffer.
(custom-buffer-create-internal): When creating the command buttons,
add the optional :notify function to enable/disable them.  Add the
buttons to the new variable custom-command-buttons.
(customize-menu-create): Notify the command buttons after creating the
Custom buffer, so they are correctly enabled/disabled.
(custom-redraw-magic, custom-notify): Notify the command buttons and
update the tool bar when changing a widget to the modified state.