Refactor image handling in the exercise submission process

Now the images are kept in a single list, instead of having two for
the files themselves and the metadata.

lib/models/exercises/exercise_submission_images.dart

@@ -0,0 +1,67 @@
+/*
+ * This file is part of wger Workout Manager <https://github.com/wger-project>.
+ * Copyright (C) 2020, 2021 wger Team
+ *
+ * wger Workout Manager is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+
+enum ImageType {
+  photo(id: 1, label: 'Photo', icon: Icons.photo_camera),
+  threeD(id: 2, label: '3D', icon: Icons.view_in_ar),
+  line(id: 3, label: 'Line', icon: Icons.show_chart),
+  lowPoly(id: 4, label: 'Low-Poly', icon: Icons.filter_vintage),
+  other(id: 5, label: 'Other', icon: Icons.more_horiz);
+
+  const ImageType({required this.id, required this.label, required this.icon});
+
+  final int id;
+  final String label;
+  final IconData icon;
+}
+
+class ExerciseSubmissionImage {
+  final File imageFile;
+
+  String? title;
+  String? author;
+  String? authorUrl;
+  String? sourceUrl;
+  String? derivativeSourceUrl;
+  ImageType type = ImageType.photo;
+
+  ExerciseSubmissionImage({
+    this.title,
+    this.author,
+    this.authorUrl,
+    this.sourceUrl,
+    this.derivativeSourceUrl,
+    this.type = ImageType.photo,
+    required this.imageFile,
+  });
+
+  Map<String, String> toJson() {
+    return {
+      'title': title ?? '',
+      'author': author ?? '',
+      'author_url': authorUrl ?? '',
+      'source_url': sourceUrl ?? '',
+      'derivative_source_url': derivativeSourceUrl ?? '',
+      'type': type.id.toString(),
+    };
+  }
+}