Set a specific disk cache dir for the web process.

* src/client/application/geary-controller.vala
  (GearyController::open_async): Pass new cache dir through to
  ClientWebView::init_web_context. Update avatar cache dir name to remove
  redundancy.

* src/client/components/client-web-view.vala
  (ClientWebView::init_web_context): Add additional cache dir para and
  update call sites. Use to construct a WebsiteDataManager
  instance. Construct a new WebContext using the data manager, and use
  that instead of the default one when creating new ClientWebView
  instances.

test/client/components/client-web-view-test-case.vala

@@ -22,6 +22,7 @@ public abstract class ClientWebViewTestCase<V> : Gee.TestCase {
         ClientWebView.init_web_context(
             this.config,
             File.new_for_path(_BUILD_ROOT_DIR).get_child("src"),
+            File.new_for_path("/tmp"), // XXX use something better here
             true
         );
         try {