Add support for snoozing alarms

Also modified time_manager to pass time by value to time callbacks as
it's a fairly small struct and it will probably be more robust.

components/display/display.c

@@ -51,9 +51,9 @@ static void show_time(unsigned hour, unsigned minute)
 	show_digit(DISPLAY_DIGIT_4, minute % 10);
 }
 
-static void update_time(const Time *time)
+static void update_time(Time now)
 {
-	show_time(time->hour, time->minute);
+	show_time(now.hour, now.minute);
 }
 
 void display_init()