Add override option for SettingsList alignment

[Llamadmiral]

SettingsList looks really bad on wide screen (such as tablets), and all the constraints for its position are only working if you constraint the size aswell, resulting in bad graphic designs.

Line in question:

flutter-settings-ui/lib/src/list/settings_list.dart

Line 64 in d8d97b2

alignment: Alignment.center,