translated Title and Alt text fix, use optionally with entitycache and entity_translation

Problem/Motivation

Currently the handling of the alt / title properties of file entities in file_entity_file_load() doesn't take in account that there's also the Entity cache module which caches entities language agnostic.
Now if you fix the File Entity module with #2553067: Language option missing for token replacements to make the alt / title property replacement language aware (probably using Entity Translation) then using Entity cache will basically revert the fix by serving the cache, language agnostic, version.
So the alt / title properties will be stuck in whatever language was used to prime the Entity cache.

Proposed resolution

Use hook_entitycache_ENTITY_TYPE_load() to re-run the alt / title property replacement in the current language whenever the entities are served by Entity cache.
To do so split the current replacement into a dedicated function (how about file_entity_set_title_alt_properties()?) so it can be re-used in both hooks.

The attached patch does exactly this.
It also implements parts of the change here: #2553067: Language option missing for token replacements

Remaining tasks

Reviews needed.

User interface changes

None.

API changes

None.

Data model changes

None.