add statusbar message upon key-hander activation (#98)

Currently when running the key-handler the statusbar shows a
"Running key-handler..." message, but there's no indication of the prefix key
being pressed.

There's a slight functional benefit of this patch in the sense
that users can visually tell if the key-handler is listening on input or if the
key-handler has been aborted or not.

commands.c

@@ -37,6 +37,7 @@ void animate(void);
 void slideshow(void);
 void set_timeout(timeout_f, int, bool);
 void reset_timeout(timeout_f);
+void handle_key_handler(bool);
 
 extern appmode_t mode;
 extern img_t img;
@@ -114,7 +115,7 @@ bool cg_toggle_bar(arg_t _)
 
 bool cg_prefix_external(arg_t _)
 {
-	extprefix = true;
+	handle_key_handler(true);
 	return false;
 }