efl.file: improve api a bit

efl.file: improve api a bit

Summary:
the previous implementation/api had a number of issues:
* "file" property contained both "file" and "key" values
  - also performed file loading operation
* "load_error" property which was specific to image objects
* no methods for controlling file loading/unloading

this patch attempts the following changes:
* split "file" property into "file" and "key" properties
  - also remove "key" from existing "mmap" property
* remove "load_error"
* directly return error codes from operations
* add "load" and "unload" methods for directly controlling load state
* add implicit file loading if file/mmap is set during construction
* rewrite all efl.file implementations to move file loading into load() method
* rewrite all usage of efl.file api based on these changes
* add C extension functions to mimic previous behavior

ref T7577

Reviewers: segfaultxavi, bu5hm4n, cedric

Reviewed By: segfaultxavi

Subscribers: vitor.sousa, #reviewers, #committers

Tags: #efl_api

Maniphest Tasks: T7577

Differential Revision: https://phab.enlightenment.org/D8018