elm: Prevent style/theme set after finalize (EO)
This is only for EO obviously. style_set and theme_set should only be
called when the object is being created, not after. On-the-fly style
changes are complex to handle and in most cases it should be easy to
simply repopulate the object after creating a new one with a new style.
There are only a few cases where style_set is used long after creation
of an object, like changing how a label slides, or in the evas 3d map
examples. Menu seems to change the hover style a lot, so rewriting it in
pure EO would need some extra work, maybe.