Genlist: Calculate the min size of content before item realize

Genlist: Calculate the min size of content before item realize

Summary:
Genlist item doesn't change its size when its content size is changed,
but its size is determined in realization.
Therefore, deferred calculations for content should be performed immediately
before swallowing it by genlist item.

Test Plan: make and run attached sample

Reviewers: jpeg, SanghyeonLee, cedric

Differential Revision: https://phab.enlightenment.org/D4705