efl.screen: add screen_scale_factor property

Summary:
ref T7590

Depends on D7966

Reviewers: segfaultxavi

Reviewed By: segfaultxavi

Subscribers: segfaultxavi, cedric, reviewers, committers

Tags: efl: api

Maniphest Tasks: T7590

Differential Revision: https://phab.enlightenment.org/D7967