* 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.