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>