Add Countdown Face

A simple countdown face using movement_schedule_background_task
so that it rings, even if the another face is active

Limited to 99 minutes and 59 seconds so that the watch isn't staying
out of deep sleep for too long

movement/movement_config.h

@@ -17,6 +17,7 @@
 #include "lis2dh_logging_face.h"
 #include "demo_face.h"
 #include "hello_there_face.h"
+#include "countdown_face.h"
 
 const watch_face_t watch_faces[] = {
     simple_clock_face,