layout: Move edje_get to legacy only

Here's the reasoning:

1. We will expose as many edje APIs as possible (and meaningful) through the elm layout class.
2. Access to internal objects is usually risky, as it allows apps to bypass EFL in some ways, leading to potentially undefined behaviours.
3. If the need arises we can still add a similar API back to EO, later.

Back to #1, it seems that the need for edje_get() was mostly to
call manual sizing functions, or the missing message_send(). I will
make sure these are accessible from the layout itself.

Ref T5315