Wire DPI setting from resize step through to processing job

Add output_dpi field to JobConfig, ProcessingJob, and Preset.
Connect the DPI SpinRow in the resize step's advanced options to
update the config value.

pixstrip-gtk/src/steps/step_resize.rs

@@ -460,6 +460,12 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
             jc.borrow_mut().resize_algorithm = row.selected();
         });
     }
+    {
+        let jc = state.job_config.clone();
+        dpi_row.connect_value_notify(move |row| {
+            jc.borrow_mut().output_dpi = row.value() as u32;
+        });
+    }
 
     scrolled.set_child(Some(&content));