Restructure Storage to new paradigm

tests/Util/SampleStorageBackend.php

@@ -22,14 +22,14 @@
 namespace Friendica\Test\Util;
 
 use Friendica\Core\Hook;
-use Friendica\Model\Storage\IWritableStorage;
+use Friendica\Core\Storage\Capability\ICanWriteToStorage;
 
 use Friendica\Core\L10n;
 
 /**
  * A backend storage example class
  */
-class SampleStorageBackend implements IWritableStorage
+class SampleStorageBackend implements ICanWriteToStorage
 {
 	const NAME = 'Sample Storage';
 
@@ -102,7 +102,7 @@ class SampleStorageBackend implements IWritableStorage
 		return $this->options;
 	}
 
-	public function __toString()
+	public function __toString(): string
 	{
 		return self::NAME;
 	}