Introduce a `selected_setting_new_value` field in `SettingsState` and
initialize it.
Add `Backspace` and `WildCard(char)` actions with an `Input` group, and
corresponding keybindings.
Update `event_to_action` to map wildcard characters to
`Action::WildCard`.
Handle backspace and character input in `settings_keybindings` to edit
the new field.
Display the edited value in the settings popup and include the new
actions in the UI.