edje: Implement EO API for message_send
Since this uses generic_value (aka Eina_Value), there is an
implicit type detection and conversion. I did not attempt to
cover all types (namely string+int and string+float and even
less sets of those) as I believe they aren't really used. Or
they most likely could be replaced by multiple messages rather
than a single message.
Note: should we pass ptr(generic_value) instead of generic_value?
The API looks a little odd in C when passing by value.