Better client/login/authState handling

flutter/lib/state/token_source.dart

@@ -0,0 +1,21 @@
+abstract class TokenSource {
+  String getToken();
+  String getUserID();
+}
+
+class DirectTokenSource implements TokenSource {
+  final String _userID;
+  final String _token;
+
+  DirectTokenSource(this._userID, this._token);
+
+  @override
+  String getUserID() {
+    return _userID;
+  }
+
+  @override
+  String getToken() {
+    return _token;
+  }
+}