Add locale to CmsSessionContext

For Visual Editing, we require a Locale in the REST service to correctly select the localized name of a document type, document field caption or hint. Adding the locale to the CmsSessionContext seems the most logical way to make the locale available to the REST service (which already uses the session's credentials).

This improvement will affect hippo-services-api (new constant and default interface method) and the hippo-cms (populate the actual Locale value).

The plan is to implement support for this off the master branch, rather than in Visual Editing feature-context.