Per Jehan, we should remove gimppickbutton-default.[ch] and
move the functions into gimppickbutton.c. Then we can
make the GimpPickButtonPrivate API actually private without
any additional public functions.
Temporarily reverting as the private struct
is shared between various OS versions of
the object and needs to be accessible.
A different approach will be needed to resolve,
and until then, I'd like the Debian pipeline to
work properly.