Clean some code

android/gradle.properties

lib/main.dart

lib/ui/painters/graph_painter.dart

lib/ui/screens/about_page.dart

lib/ui/screens/demo_page.dart

lib/ui/screens/graph_page.dart

lib/ui/screens/settings_page.dart

lib/ui/skeleton.dart

lib/ui/widgets/app_bar.dart

lib/ui/widgets/bottom_nav_bar.dart

lib/ui/widgets/header_app.dart

lib/ui/widgets/settings_form.dart

pubspec.lock

pubspec.yaml