CMS-2186 Schema Manager: Make "Schema edit" use new REST API

When handling app_browse.EditSchemaEvent use new REST API for each schema type.
 * GetContentTypeBy..., GetMixinBy..., GetRelationshipTypeBy....
And of course, change each XxxWizardPanel accept the new domain classes:
 * api_schema_content.ContentType, api_schema_relationshiptype, api_mixin.Mixin

Also RelationshipTypeResource was refactored: method get was broken on two methods: get and getConf