window: Bind headerbar opacity to controls

No purpose yet, but will help when we want to implement
fading. Need to have support for closures in Rust first.

data/gtk/image_view.ui

@@ -14,7 +14,7 @@
           </object>
         </child>
         <child type="overlay">
-          <object class="GtkBox">
+          <object class="GtkBox" id="controls">
             <property name="vexpand">True</property>
             <child>
               <object class="GtkButton" id="prev_button">

data/gtk/window.ui

@@ -15,6 +15,11 @@
           <object class="GtkHeaderBar" id="headerbar">
             <property name="css-classes">titlebar</property>
             <property name="show_title_buttons" bind-source="IvWindow" bind-property="fullscreened" bind-flags="sync-create|invert-boolean"/>
+            <binding name="opacity">
+              <lookup type="GtkWidget" name="opacity">
+                <lookup name="controls">image_view</lookup>
+              </lookup>
+            </binding>
             <child type="end">
               <object class="GtkMenuButton" id="menu_button">
                 <property name="icon-name">open-menu-symbolic</property>

src/widgets/image_view.rs

@@ -44,6 +44,8 @@ mod imp {
         #[template_child]
         pub picture: TemplateChild<gtk::Picture>,
         #[template_child]
+        pub controls: TemplateChild<gtk::Box>,
+        #[template_child]
         pub popover: TemplateChild<gtk::PopoverMenu>,
         #[template_child]
         pub click_gesture: TemplateChild<gtk::GestureClick>,
@@ -103,6 +105,13 @@ mod imp {
                         None,
                         glib::ParamFlags::READABLE,
                     ),
+                    glib::ParamSpec::new_object(
+                        "controls",
+                        "Controls",
+                        "The controls for the image view",
+                        gtk::Box::static_type(),
+                        glib::ParamFlags::READABLE,
+                    ),
                 ]
             });
 
@@ -133,6 +142,7 @@ mod imp {
             match psec.name() {
                 "popover-menu-model" => self.popover_menu_model.borrow().to_value(),
                 "filename" => self.filename.borrow().to_value(),
+                "controls" => self.controls.to_value(),
                 _ => unimplemented!(),
             }
         }