* IBehaviour extends interfaces IHeaderContributor and
IBodyOnloadContributor so that they can make contributions
for _javascript_ etc. This is needed for ajax implementations
anyway, but could certainly come in handy for a lot of
custom behaviours.
Why extend it directly?
Is an IBehaviour really an extention on both?
Don't think so. (for example an AttributeModifier by itself doesn't use any of the other 2)
And if somebody makes a implementation it can easily implement 1,2 or 3 of them.
