explorer: move files state from fileStorage

This state should be exclusivly used to for displaying files in the
explorer, so the state should be in the explorer branch of the
reducer tree.

src/explorer/renameFileDialog/RenameFileDialog.tsx

@@ -24,7 +24,7 @@ const RenameFileDialog: React.VFC = () => {
     const [baseName, extension] = oldName.split(/(\.\w+)$/);
 
     const [newName, setNewName] = useState(baseName);
-    const files = useSelector((s) => s.fileStorage.files);
+    const files = useSelector((s) => s.explorer.files);
     const result = validateFileName(
         newName,
         extension,