input: Add a scroll_factor config option

This is not a setting handled by libinput; we have to scale the
scrolling ourselves.

This should match the behavior of the `scroll_factor` defined in
sway-input(5).

cosmic-comp-config/src/input.rs

@@ -42,6 +42,7 @@ pub struct ScrollConfig {
     pub method: Option<ScrollMethod>,
     pub natural_scroll: Option<bool>,
     pub scroll_button: Option<u32>,
+    pub scroll_factor: Option<f64>,
 }
 
 #[derive(Copy, Clone, Debug, Serialize, Deserialize)]