Add the make_modal method

protocol/pantheon-desktop-shell-v1.xml

@@ -128,5 +128,13 @@
         by the compositor.
       </description>
     </request>
+
+    <request name="make_modal">
+      <description summary="requests to make a surface system modal">
+        This will block all user input outside the surface and most system shortcuts.
+      </description>
+
+      <arg name="dim" type="uint" summary="1 to dim, 0 to not dim"/>
+    </request>
   </interface>
 </protocol>

protocol/pantheon-desktop-shell.vapi

@@ -70,5 +70,6 @@ namespace PantheonDesktop {
         public void set_keep_above ();
         public void make_centered ();
         public void focus ();
+        public void make_modal (uint dim);
     }
 }

src/ExtendedBehavior.vala

@@ -84,4 +84,21 @@ public interface PantheonWayland.ExtendedBehavior : Gtk.Widget, Gtk.Native {
             warning ("Couldn't focus: ExtendedBehavior surface was null. Did you forget to call connect_to_shell?");
         }
     }
+
+    /**
+     * Tells the wm to make this window modal. The window will be displayed above
+     * all other windows. All user input outside this window and most system shortcuts
+     * will be blocked until this window is closed.
+     * If dim is true the background will be dimmed.
+     * This should only be called after {@link connect_to_shell}.
+     * This is only allowed for centered windows, so {@link make_centered} has to be called first.
+     */
+    public void make_modal (bool dim) {
+        unowned PantheonDesktop.ExtendedBehavior? extended_behavior = get_data ("-pantheon-wayland-extended-behavior");
+        if (extended_behavior != null) {
+            extended_behavior.make_modal ((uint) dim);
+        } else {
+            warning ("Couldn't make modal: ExtendedBehavior surface was null. Did you forget to call connect_to_shell?");
+        }
+    }
 }