Bug #709396 - Quoted text truncated when "Keep signature above" is set

The GtkComboBox::changed signal is not called only on active-id change,
but also on any content change, including each row add/change/removal
into its model. These signals are delivered on the main thread, thus
the result of e_mail_signature_combo_box_load_selected() is delivered
later, which means that a call like gtk_list_store_clear() queues as
many 'load_selected' operations as is the list items count. Such thing
confused GtkHTML editor enough to garble the message body. This change
is kind of workaround, which avoids unnecessary signature updates.