Add accessible labels to sliders and watermark position grid

Screen readers now announce the purpose and range of brightness,
contrast, saturation, and compression quality sliders. The
watermark position grid frame also has a descriptive label.

pixstrip-gtk/src/steps/step_watermark.rs

@@ -120,6 +120,9 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         .halign(gtk::Align::Center)
         .build();
     grid_frame.set_child(Some(&grid));
+    grid_frame.update_property(&[
+        gtk::accessible::Property::Label("Watermark position grid. Select where the watermark appears on the image."),
+    ]);
 
     let mut first_button: Option<gtk::ToggleButton> = None;
     let buttons: Vec<gtk::ToggleButton> = position_names.iter().enumerate().map(|(i, name)| {