Core: Minor fixes and improvements
* User field: Added new 'Return Format' setting (Array, Object, ID)
* Core: Added basic compatibility with Gutenberg - values now save
* Core: Fixed bug affecting the loading of fields on new Menu Items
* Core: Removed private ('show_ui' => false) post types from the 'Post Type' location rule choices
* Core: Minor fixes and improvements
* Language: Updated French translation - thanks to Maxime Bernard-Jacquet