Allow determining when JS has finished loading in ClientWebView.

* src/client/components/client-web-view.vala (ClientWebView): Add
  is_content_loaded property and content_loaded signal, update and fire
  when getting a contentLoaded message from the WebProcess.

* ui/client-web-view.js: Fire the contentLoaded message when loading is
  complete. Add ClientPageStateTest test case to ensure it is working
  fine.

* test/client/components/client-web-view-test-case.vala
  (ClientWebViewTestCase::load_body_fixture): Use is_content_loaded
  rather than is_loading as the test for loading having finished, since
  we're actually interested in when the JS has finished loaded, not the
  resources.

test/js/client-page-state-test.vala

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2017 Michael Gratton <mike@vee.net>
+ *
+ * This software is licensed under the GNU Lesser General Public License
+ * (version 2.1 or later). See the COPYING file in this distribution.
+ */
+
+class ClientPageStateTest : ClientWebViewTestCase<ClientWebView> {
+
+
+    public ClientPageStateTest() {
+        base("ClientPageStateTest");
+        add_test("content_loaded", content_loaded);
+
+        try {
+            ClientWebView.load_scripts();
+        } catch (Error err) {
+            assert_not_reached();
+        }
+
+    }
+
+    public void content_loaded() {
+        bool content_loaded_triggered = false;
+        this.test_view.content_loaded.connect(() => {
+                content_loaded_triggered = true;
+            });
+
+        assert(!this.test_view.is_content_loaded);
+
+        // XXX sketchy - this call will never return if the thing we
+        // are testing does not work
+        load_body_fixture("OHHAI");
+
+        assert(this.test_view.is_content_loaded);
+        assert(content_loaded_triggered);
+    }
+
+    protected override ClientWebView set_up_test_view() {
+        WebKit.UserScript test_script;
+        test_script = new WebKit.UserScript(
+            "var geary = new PageState()",
+            WebKit.UserContentInjectedFrames.TOP_FRAME,
+            WebKit.UserScriptInjectionTime.START,
+            null,
+            null
+        );
+
+        ClientWebView view = new ClientWebView(this.config);
+        view.get_user_content_manager().add_script(test_script);
+        return view;
+    }
+
+}