Add Activity actions buttons

CHANGELOG.md

lib/flutter_toolbox.dart

lib/settings/application_settings_form.dart

lib/settings/application_settings_theme_card.dart

lib/widgets/activity/button_delete_saved_activity.dart

lib/widgets/activity/button_quit_activity.dart

lib/widgets/activity/button_resume_saved_activity.dart

lib/widgets/activity/button_start_new_activity.dart

lib/widgets/styled_button.dart

pubspec.yaml