Bug #599792 - Anjal composer's Send button doesn't work after pressed Save Drafts button

This is due to an old hack that hiding a composer means we're closing
it so save_draft_done() destroys the composer after saved draft. But
in Anjal, the composer widget is always hidden (since the editor is
reparented to the tab), and will be wrongly destroyed by
save_draft_done() when you clicked "Save Draft" button.

This patch improved the old hack, by adding a new API
e_msg_composer_request_close() that can be used to request closing a
composer (so the old hack is no longer needed). Internally,
composer->priv->application_exiting is used to store this exiting
status.

So by this we no longer use a composer's visibility to check whether
we're to close it. When you no longer need a composer after saved
draft, call e_msg_composer_request_close() before sending the
save-draft signal.