Port Language field to Drupal 8

This module is ported to Drupal 8.

For now, Languagefield only allows to select a subset from the list of 'predefined languages'.

Functionalities

Languagefield introduced a Field type, Widgets and Formatters.

Things to do (patches are welcome :-) ):

• Add option to add self-defined languages in LanguageItem::storageSettingsForm(); fixed as per the patch in comment #11
• Add option to add both predefined and self-defined languages in LanguageItem; see #2116687: Using new (custom) languages AND predefined languages
• Port function hook_field_widget_info_alter(); see #2270891-4: Language field should depend on Options
• #2636144: [D8-port] add languageicons support
• #2619446: Integration with Views (D8);
• Extend hook_help, explaining the new StorageSettingsForm (or on the project page, even if this is core behaviour.)
• function LanguageItem::getNativeName() should be added to \Drupal\Core\Language\Language.

Things removed from module:

• Port function hook_field_prepare_view() to LanguageFormatter::prepareView();
• the 'group help' in LanguageItem::storageSettingsForm(); - a link to a list should suffice; a fresh patch is welcome.
• Feeds support; - a fresh patch is welcome.

Things removed from Drupal core:
- disabled languages are no longer possible. See:
#1314250: Allow filtering/configuration of which languages apply to what (UI, nodes, files, etc)
#1539072: Support for disabled languages broken, drop it

New features added:
#2358913: Add options for site/user/interface language as default value

The scope of this module might change, due to ongoing changes in D8 core:
- #2230637: Create a Language field widget and the related formatter
- #2397729: Content language settings cannot be configured for non-translatable types on the one page configuration form
- #2143729: Entity definitions miss a language entity key

This module implements a FieldItem.