更改安卓端的文件访问方式，优化导入逻辑 (#64)

* Refactor import function & Allow import local comics without copying them to local path.

* android: use file_picker instead, support directory access for android 10

* Improve import logic

* Fix sql query.

* Add ability to remove invalid favorite items.

* Perform sort before choosing cover

* Revert changes of "use file_picker instead".

* Try catch on "check update"

* Added module 'flutter_saf'

* gitignore

* remove unsupported arch in build.gradle

* Use flutter_saf to handle android's directory and files, improve import logic.

* revert changes of 'requestLegacyExternalStorage'

* fix cbz import

* openDirectoryPlatform

* Remove double check on source folder

* use openFilePlatform

* remove unused import

* improve local comic's path handling

* bump version

* fix pubspec format

* return null when comic folder is empty

lib/foundation/image_provider/cached_image.dart

@@ -3,6 +3,7 @@ import 'dart:io';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:venera/network/images.dart';
+import 'package:venera/utils/io.dart';
 import 'base_image_provider.dart';
 import 'cached_image.dart' as image_provider;
 
@@ -24,7 +25,7 @@ class CachedImageProvider
   @override
   Future<Uint8List> load(StreamController<ImageChunkEvent> chunkEvents) async {
     if(url.startsWith("file://")) {
-      var file = File(url.substring(7));
+      var file = openFilePlatform(url.substring(7));
       return file.readAsBytes();
     }
     await for (var progress in ImageDownloader.loadThumbnail(url, sourceKey, cid)) {