Randomly load Sleep Screen from /sleep/*bmp (if exists). (#71)

Load a random sleep screen. 

Only works for ".bmp" (not ".BMP"), but I think that's OK (we do the
same for EPUBs).

src/activities/boot_sleep/SleepActivity.h

@@ -11,4 +11,5 @@ class SleepActivity final : public Activity {
  private:
   void renderDefaultSleepScreen() const;
   void renderCustomSleepScreen(const Bitmap& bitmap) const;
+  void renderPopup(const char* message) const;
 };