Add blank sleep screen option (#242)

## Summary

Very small change to add a blank ("None") sleep screen option, for those
who prefer a clean aesthetic.

Tested on X4 device.

src/CrossPointSettings.h

@@ -16,7 +16,7 @@ class CrossPointSettings {
   CrossPointSettings& operator=(const CrossPointSettings&) = delete;
 
   // Should match with SettingsActivity text
-  enum SLEEP_SCREEN_MODE { DARK = 0, LIGHT = 1, CUSTOM = 2, COVER = 3 };
+  enum SLEEP_SCREEN_MODE { DARK = 0, LIGHT = 1, CUSTOM = 2, COVER = 3, BLANK = 4 };
 
   // Status bar display type enum
   enum STATUS_BAR_MODE { NONE = 0, NO_PROGRESS = 1, FULL = 2 };