refactor: Move widgets to lib

lib/widgets/adaptive_flat_button.dart

@@ -0,0 +1,34 @@
+import 'package:fluffychat/utils/platform_infos.dart';
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
+
+class AdaptiveFlatButton extends StatelessWidget {
+  final String label;
+  final Color textColor;
+  final Function onPressed;
+
+  const AdaptiveFlatButton({
+    Key key,
+    @required this.label,
+    this.textColor,
+    this.onPressed,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    if (PlatformInfos.isCupertinoStyle) {
+      return CupertinoDialogAction(
+        onPressed: onPressed,
+        textStyle: textColor != null ? TextStyle(color: textColor) : null,
+        child: Text(label),
+      );
+    }
+    return TextButton(
+      onPressed: onPressed,
+      child: Text(
+        label,
+        style: TextStyle(color: textColor),
+      ),
+    );
+  }
+}