Watch face selection at build time

Do not assign specific values to the fields of the enum WatchFace.

src/displayapp/Apps.h

@@ -44,12 +44,12 @@ namespace Pinetime {
     };
 
     enum class WatchFace : uint8_t {
-      Digital = 0,
+      Digital,
-      Analog = 1,
+      Analog,
-      PineTimeStyle = 2,
+      PineTimeStyle,
-      Terminal = 3,
+      Terminal,
-      Infineat = 4,
+      Infineat,
-      CasioStyleG7710 = 5,
+      CasioStyleG7710,
     };
 
     template <Apps>