EShellBackend: Respond to EShell::prepare-for-quit signals.

Listen for "prepare-for-quit" signals from the shell and inhibit
shutdown until all the activities we're tracking are finalized.

Also, add a couple supporting functions:

  gboolean  e_shell_backend_is_busy     (EShellBackend *shell_backend);
  void      e_shell_backend_cancel_all  (EShellBackend *shell_backend);

These will eventually replace mail_msg_active() and mail_cancel_all().

doc/reference/shell/tmpl/e-shell-backend.sgml

@@ -34,6 +34,11 @@ EShellBackend
 @eshellbackend: the object which received the signal.
 @arg1: 
 
+<!-- ##### ARG EShellBackend:busy ##### -->
+<para>
+
+</para>
+
 <!-- ##### FUNCTION e_shell_backend_compare ##### -->
 <para>