#5 button set_member and get_member

[Evan Meakyl]

this patch adds to the button object the following member:
_x, _y (writting and reading)
_alpha (writting and reading)
_visible (writting and reading)
_width, _height (reading)

and correct a small problem, if your button should play
sound but that the client didn't set any handler

I am working on the other button members.

[Evan Meakyl]

set_member and get_member diffs