EMailConfigWebView is an extension that configures EWebView instances.
This used to cover both the email formatter and the email composer, but
with the email formatter now using WebKit/GTK+ and the email composer
still using GtkHtml, the composer was left unconfigured.
Add an EMailConfigWebViewGtkHTML extension that targets EWebViewGtkHTML
instances and applies the relevant settings, and remove all the GtkHTML
contortions from EMailConfigWebView.
d610562650