Module's entity handlers for Entity Reference fields that points to nodes and terms are wrong

Problem/Motivation

Both EntityReference_SelectionHandler_Translation_Generic_node and EntityReference_SelectionHandler_Translation_Generic_taxonomy_term are wrong. With it's current code they only work on Entity Reference (ER) in one case:

Node context:
ER field that points to nodes: OK
ER field that ponts to taxonomies: KO *

Taxonomy context:
ER field that points to nodes: KO
ER field that ponts to taxonomies: KO

The problem is that in EntityReference_SelectionHandler_Translation_Generic_node the function i18n_node_i18n_context_language is called to obtain the language of the node in context but the ER field could be in a taxonomy context too!

The same problem happens in EntityReference_SelectionHandler_Translation_Generic_taxonomy_term: the function i18n_taxonomy_i18n_context_language is called to obtain the language of the taxonomy in context but the ER field could be in a node context too!

* This notice shows up:

Notice: Undefined index: locale in EntityReference_SelectionHandler_Translation_Generic_taxonomy_term->entityFieldQueryAlter() (line 95 of /sites/all/modules/contrib/translated_entityreference/plugins/selection/EntityReference_SelectionHandler_Translation_Generic.class.php).

It is highly recommended to also apply patch of issue 2762971.

Proposed resolution

Call both i18n_node_i18n_context_language and i18n_taxonomy_i18n_context_language in both handlers and then use the result of two which is not empty.

Fix EntityReference_SelectionHandler_Translation_Generic_taxonomy_term (it lacks getReferencableEntities implementation).

Remaining tasks

None. Test this by editing nodes and taxonomies with ER fields.