Move reusable code from mobile to common

This will be useful for a future desktop application.

common/lib/src/platform.dart

@@ -0,0 +1,76 @@
+/// Object representing a document in Storage Access Framework terms.
+class Document {
+  /// Unique ID for the document.
+  ///
+  /// The platform implementation thould be able to get the content of the
+  /// document based on this value.
+  final String id;
+
+  /// Name of the document (i.e. file name).
+  final String name;
+
+  /// Document ID of the parent document.
+  final String parent;
+
+  /// Whether this document represents a directory.
+  final bool isDirectory;
+
+  Document({
+    this.id,
+    this.name,
+    this.parent,
+    this.isDirectory,
+  });
+
+  // Use Map<dynamic, dynamic> here, as we get casting errors otherwise. This
+  // won't be typesafe anyway.
+  Document.fromJson(Map<dynamic, dynamic> json)
+      : id = json['id'],
+        name = json['name'],
+        parent = json['parent'],
+        isDirectory = json['isDirectory'];
+}
+
+/// Platform dependent code for the Musicus backend.
+abstract class MusicusPlatform {
+  /// An identifier for the root directory of the music library.
+  ///
+  /// This will be the string, that is stored as musicLibraryPath in the
+  /// settings object.
+  String basePath;
+
+  MusicusPlatform();
+
+  /// This will be called, when the music library path was changed.
+  void setBasePath(String path) {
+    basePath = path;
+  }
+
+  /// Get all documents in a directory.
+  ///
+  /// [parentId] will be the ID of the directory document. If [parentId] is
+  /// null, the children of the root directory will be returned.
+  Future<List<Document>> getChildren(String parentId);
+
+  /// Read the contents of a document by ID.
+  Future<String> readDocument(String id);
+
+  /// Read from a document by name.
+  /// 
+  /// [parentId] is the document ID of the parent directory.
+  Future<String> readDocumentByName(String parentId, String fileName);
+
+  /// Get a string identifying a document.
+  ///
+  /// [parentId] is the document ID of the parent directory. The return value
+  /// should be a string, that the playback object can use to find and play the
+  /// file. It will be included in [InternalTrack] objects by the music
+  /// library.
+  Future<String> getIdentifier(String parentId, String fileName);
+
+  /// Write to a document by name.
+  ///
+  /// [parentId] is the document ID of the parent directory.
+  Future<void> writeDocumentByName(
+      String parentId, String fileName, String contents);
+}