Add e_activity_get_last_known_text().

Evolution is still occasionally getting stuck on shutdown, and although
the evolution-shell log domain shows debug messages for activities that
are preventing shutdown, they frequently look like this:

(evolution:13534): evolution-shell-DEBUG: 5 active 'mail' activities:
(evolution:13534): evolution-shell-DEBUG: * (no description)
(evolution:13534): evolution-shell-DEBUG: * (no description)
(evolution:13534): evolution-shell-DEBUG: * (no description)
(evolution:13534): evolution-shell-DEBUG: * (no description)
(evolution:13534): evolution-shell-DEBUG: * (no description)

I think the lack of descriptions is from CamelOperations popping all
their pushed messages, which is correct behavior but doesn't help us
debug the problem.

e_activity_get_last_known_text() returns the most recent _non-empty_
text value set on the EActivity.  So our debug message can fall back
to that if the EActivity has no description at shutdown:

(evolution:13534): evolution-shell-DEBUG: * (was "blah, blah, blah")

1 files changed, 1 insertions, 0 deletions

diff --git a/e-util/e-activity.h b/e-util/e-activity.h
index 9309fe1fb0..1c2cc4e7d3 100644
--- a/e-util/e-activity.h
+++ b/e-util/e-activity.h
@@ -90,6 +90,7 @@ void		e_activity_set_state		(EActivity *activity,
 const gchar *	e_activity_get_text		(EActivity *activity);
 void		e_activity_set_text		(EActivity *activity,
 						 const gchar *text);
+const gchar *	e_activity_get_last_known_text	(EActivity *activity);
 gboolean	e_activity_handle_cancellation	(EActivity *activity,
 						 const GError *error);