Put breadcrumbs on overlay instead of inserting to buffer (bug#5809).

Put breadcrumbs on overlay instead of inserting to buffer (bug#5809).

* info.el (Info-find-node-2): Comment out code that skips
breadcrumbs line.
(Info-mouse-follow-link): New command.
(Info-link-keymap): New keymap.
(Info-breadcrumbs): Rename from `Info-insert-breadcrumbs'.
Return a string with links instead of inserting breadcrumbs
to the Info buffer.
(Info-fontify-node): Comment out code that inserts breadcrumbs.
Instead of putting the `invisible' text property over the Info
header, make an overlay over the Info header with the `invisible'
property and `after-string' set to the string returned by
`Info-breadcrumbs'.