Clean up how composer loads content into its web view.

The main gist of this is to ensure that the composer's widgets are
constructed seperately to loading its content, and that we only ever call
ComposerWebView::load_html precisely once per composer instance.

* src/client/composer/composer-widget.vala: Remove referred message,
  quote text and draft flag param from constructor signature, move any
  calls that loaded data from them to new load method. Don't load
  anything into the editor here. Make loading the signature file async,
  and call new ComposerWebView::updateSignature method on the editor to
  update it.
  (ComposerWidget::load): New async message for loading content into the
  composer. Move related code from the constructor and GearyController
  here, make methods that were previously public for that private
  again. Tidy up calls a bit now that we have a single place from which
  to do it all, and can understand the process a bit better.
  (ComposerWidget::on_editor_key_press_event): Don't reload the editor to
  remove the quoted text, call new ComposerWebView::delete_quoted_message
  method on it instead.

* src/client/composer/composer-web-view.vala
  (ComposerWebView): Add ::delete_quoted_message ::update_signature
  methods, thunk to JS.
  (ComposerWebView::load_html): Add quote and is_draft parameters,
  construct HTML for the composer using apporporate spacing here, instead
  of relying on all the disparate parts from doing the right thing.

* src/client/application/geary-controller.vala
  (GearyController::create_compose_widget_async): Load composer content
  after adding it to the widget hierarchy, set focus only after
  everything is set up.

* src/engine/rfc822/rfc822-utils.vala (quote_email_for_reply,
  quote_email_for_forward): Don't add extra padding around quoted parts -
  let callers manage their own whitespace.

* test/client/components/client-web-view-test-case.vala
  (TestCase:load_body_fixture): Make HTML param non-nullable, update
  subclasses.

* ui/composer-web-view.js (ComposerPageState): Add ::updateSignature and
  ::deleteQuotedMessage method stubs.

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

@@ -191,8 +191,8 @@ class ComposerPageStateTest : ClientWebViewTestCase<ComposerWebView> {
         return new ComposerWebView(this.config);
     }
 
-    protected override void load_body_fixture(string? html = null) {
-        this.test_view.load_html(html, null, false);
+    protected override void load_body_fixture(string html = "") {
+        this.test_view.load_html(html, "", "", false, false);
         while (this.test_view.is_loading) {
             Gtk.main_iteration();
         }