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().
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