(apropos-symbol): Add `skip' property.

(apropos-symbol): Add `skip' property.
(apropos-function, apropos-macro, apropos-command)
(apropos-variable, apropos-face, apropos-group, apropos-widget)
(apropos-plist): New button types.
(apropos-label-properties): Variable removed.
(apropos-print): Pass button-type to apropos-print-doc, rather
than help function and label text.
(apropos-print-doc): Remove ACTION and STR args, add TYPE arg.
Get button label from TYPE.