efl.file_save: rework save flags

efl.file_save: rework save flags

Summary:
instead of passing a string which requires reading docs to know which
arbitrary string key=value sets can be passed, use an extensible struct
which contains more easily referenced values

ref T7672

Depends on D8035

Reviewers: segfaultxavi

Reviewed By: segfaultxavi

Subscribers: segfaultxavi, cedric, #reviewers, #committers

Tags: #efl

Maniphest Tasks: T7672

Differential Revision: https://phab.enlightenment.org/D8039