Eo: Add eo_finalize. A func that's called at the end of eo_add.

Eo: Add eo_finalize. A func that's called at the end of eo_add.

This function lets you hook at the end of eo_add and override it for a
class. This is essentially the first step towards killing custom
constructors. Instead of having a custom constructor, you should just
do:
eo_add(CLASS, parent, a_set(3), b_set("eou"));
eo_constructor is called at the beginning for pre-init things.
eo_finalize is called at the end, for actually finalizing and doing
things. This cleans up the API and possibly saves a lot of things that
would have been stupid and slow in the past, like loading an elm widget
with an existing theme, and then changing the theme.

** This breaks Eo ABI, please recompile elementary and everything else that
creates eo objects.

@feature