idle: added "update" event

Some clients have visual feedback for "database update is running".
Using the "database" idle event is unreliable, because it is only
emitted when the database was actually modified.  This patch adds the
"update" event, which is emitted when the update is started, and again
when the update is finished, disregarding whether it has been
modified.

src/idle.c

@@ -40,6 +40,7 @@ static const char *const idle_names[] = {
 	"output",
 	"options",
 	"sticker",
+	"update",
         NULL
 };