Updates Stopwatch class to use internal state enum

Marlin/stopwatch.h

@@ -28,12 +28,6 @@
 // Print debug messages with M111 S2 (Uses 156 bytes of PROGMEM)
 //#define DEBUG_STOPWATCH
 
-enum StopwatchState {
-  STOPWATCH_STOPPED,
-  STOPWATCH_RUNNING,
-  STOPWATCH_PAUSED
-};
-
 /**
  * @brief Stopwatch class
  * @details This class acts as a timer proving stopwatch functionality including
@@ -41,7 +35,13 @@ enum StopwatchState {
  */
 class Stopwatch {
   private:
-    StopwatchState state;
+    enum State {
+      STOPPED,
+      RUNNING,
+      PAUSED
+    };
+
+    Stopwatch::State state;
     millis_t accumulator;
     millis_t startTimestamp;
     millis_t stopTimestamp;