Insufficient functionality of yank-excluded-properties

[Kim F. Storm]

Try the following: 

Do C-h f help RET and copy the contents of the *Help* buffer to
another buffer.  The mouse highlight of the help buttons is still
active, i.e. the mouse-face property hasn't been removed.

The reason is that the `category' text property is copied.
In this specific case, the category is `help-function-def-button'
which has a mouse-face property.

One option is to add `category' to yank-excluded-properties, but
that's probably too dramatic.

A better solution might be for remove-list-of-text-properties
to remove a `category' text property if that category specifies
one of the excluded text properties in the list.