* mouse-sel.el (mouse-sel-follow-link-p): Use event position

* mouse-sel.el (mouse-sel-follow-link-p): Use event position
instead of buffer position for `mouse-on-link-p'.

* mouse.el (mouse-posn-property): New function looking up the
properties at a click position in overlays and text properties in
either buffer or strings.
(mouse-on-link-p): Use `mouse-posn-property' to streamline lookup
of both `follow-link' as well as `mouse-face' properties.
(mouse-drag-track): Check `mouse-on-link-p' on event position, not
buffer position.

* help.el (describe-key-briefly): When reading a down-event on
mode lines or scroll bar, swallow the following up event, too.
Use the new mouse sensitity of `key-binding' for lookup.
(describe-key): The same here.