evas: add common part of savers and loaders.

evas: add common part of savers and loaders.

Summary:
Common part will make savers and loaders shorter and easier for understanding and refactoring.
https://phab.enlightenment.org/T2713 - due to this task.
Should be merged after https://phab.enlightenment.org/D3030

Should be merged to start adding refactored savers and loaders.

Reviewers: Hermet, raster, Oleksander, cedric

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D3038

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>