Pull the util method to serialize and deserialize a collection type into the Collection class

lib/db/collections_db.dart

@@ -98,7 +98,7 @@ class CollectionsDB {
     row[columnEncryptedKey] = collection.encryptedKey;
     row[columnKeyDecryptionNonce] = collection.keyDecryptionNonce;
     row[columnName] = collection.name;
-    row[columnType] = typeToString(collection.type);
+    row[columnType] = Collection.typeToString(collection.type);
     row[columnEncryptedPath] = collection.encryptedPath;
     row[columnPathDecryptionNonce] = collection.pathDecryptionNonce;
     row[columnCreationTime] = collection.creationTime;
@@ -112,7 +112,7 @@ class CollectionsDB {
       row[columnEncryptedKey],
       row[columnKeyDecryptionNonce],
       row[columnName],
-      typeFromString(row[columnType]),
+      Collection.typeFromString(row[columnType]),
       row[columnEncryptedPath],
       row[columnPathDecryptionNonce],
       int.parse(row[columnCreationTime]),

lib/models/collection.dart

@@ -116,26 +116,26 @@ class Collection {
         pathDecryptionNonce.hashCode ^
         creationTime.hashCode;
   }
-}
 
-CollectionType typeFromString(String type) {
-  switch (type) {
-    case "folder":
-      return CollectionType.folder;
-    case "favorites":
-      return CollectionType.favorites;
+  static CollectionType typeFromString(String type) {
+    switch (type) {
+      case "folder":
+        return CollectionType.folder;
+      case "favorites":
+        return CollectionType.favorites;
+    }
+    return CollectionType.album;
   }
-  return CollectionType.album;
-}
 
-String typeToString(CollectionType type) {
-  switch (type) {
-    case CollectionType.folder:
-      return "folder";
-    case CollectionType.favorites:
-      return "favorites";
-    default:
-      return "album";
+  static String typeToString(CollectionType type) {
+    switch (type) {
+      case CollectionType.folder:
+        return "folder";
+      case CollectionType.favorites:
+        return "favorites";
+      default:
+        return "album";
+    }
   }
 }