[webkit-efl] Use of pimpl idiom for Ewk classes

[Kenneth Rohde Christiansen]

> Performance-wise, I don't see much difference. With pimpl, we have an extra
> pointer dereferencing in the C functions and with the alternative, we have
> an extra function call.

Actually the function call should be more expensive than a simple
dereferencing. With a private class it also encapsulates the private
code quite nicely.

Kenneth