SlidingView: Stop using gtk_widget_get_allocation

It got deprecated. Hopefully "width" does what we need.

src/widgets/sliding_view.rs

@@ -276,9 +276,7 @@ mod imp {
 
         fn distance(&self) -> f64 {
             let obj = self.obj();
-            // TODO: Fix deprecation
-            #[allow(deprecated)]
-            let width = obj.allocation().width();
+            let width = obj.width();
 
             width as f64 + self.page_spacing(width) as f64
         }