Merge branch 'master' into version_4

# Conflicts:
#	README.md
#	example/lib/main.dart
#	lib/src/fields/form_builder_checkbox.dart
#	lib/src/fields/form_builder_checkbox_group.dart
#	lib/src/fields/form_builder_checkbox_list.dart
#	lib/src/fields/form_builder_chips_choice.dart
#	lib/src/fields/form_builder_chips_filter.dart
#	lib/src/fields/form_builder_chips_input.dart
#	lib/src/fields/form_builder_color_picker.dart
#	lib/src/fields/form_builder_country_picker.dart
#	lib/src/fields/form_builder_date_range_picker.dart
#	lib/src/fields/form_builder_date_time_picker.dart
#	lib/src/fields/form_builder_dropdown.dart
#	lib/src/fields/form_builder_image_picker.dart
#	lib/src/fields/form_builder_phone_field.dart
#	lib/src/fields/form_builder_radio.dart
#	lib/src/fields/form_builder_radio_group.dart
#	lib/src/fields/form_builder_range_slider.dart
#	lib/src/fields/form_builder_rate.dart
#	lib/src/fields/form_builder_segmented_control.dart
#	lib/src/fields/form_builder_signature_pad.dart
#	lib/src/fields/form_builder_slider.dart
#	lib/src/fields/form_builder_stepper.dart
#	lib/src/fields/form_builder_switch.dart
#	lib/src/fields/form_builder_text_field.dart
#	lib/src/fields/form_builder_touch_spin.dart
#	lib/src/form_builder.dart
#	lib/src/form_builder_custom_field.dart
#	lib/src/form_builder_field_option.dart
#	lib/src/widgets/grouped_radio.dart

Author: danvick

lib/src/widgets/grouped_checkbox.dart

@@ -3,7 +3,7 @@ import 'package:flutter_form_builder/flutter_form_builder.dart';
 
 class GroupedCheckbox<T> extends StatefulWidget {
   /// A list of string that describes each checkbox. Each item must be distinct.
-  final List<FormBuilderFieldOption<T>> options;
+  final List<FormBuilderFieldOption> options;
 
   /// A list of string which specifies automatically checked checkboxes.
   /// Every element must match an item from itemList.
@@ -205,7 +205,7 @@ class GroupedCheckbox<T> extends StatefulWidget {
   }) : super(key: key);
 
   @override
-  _GroupedCheckboxState<T> createState() => _GroupedCheckboxState<T>();
+  _GroupedCheckboxState createState() => _GroupedCheckboxState();
 }
 
 class _GroupedCheckboxState<T> extends State<GroupedCheckbox<T>> {