Composer.Widget: Split email body editing code out into separate widget

Create a new Composer.Editor widget and move all body web view and
action bar related code from the main widget there.

This helps to clearly delineate concerns of the two classes, it
substantially reduces the complexity of the main widget, and should
reduce the odds of further breakage like that fixed by the previous
commit less likely in the future.

src/client/application/application-client.vala

@@ -409,6 +409,7 @@ public class Application.Client : Gtk.Application {
         );
 
         MainWindow.add_accelerators(this);
+        Composer.Editor.add_accelerators(this);
         Composer.Widget.add_accelerators(this);
         Components.Inspector.add_accelerators(this);
         Components.PreferencesWindow.add_accelerators(this);