bug#12817: 24.2; The button library does not work on the header line

[Stefan Monnier]

> The attached patch allows inserting buttons into the header-line as well
> as the mode-line.  I haven't tested it much but it appears to work.
> As you suggested it doesn't use a function such as
> `header-button-format'.  Instead `make-text-button' was changed to
> return a useful value when BEG is a string: the propertized string.

Thanks, installed, after simplifying it a bit to just use STRING rather
than (STRING . STRING-POS).


        Stefan