move Period enum into its own file

PomoTime/Period.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace PomoTime
+{
+    public static class PeriodExtensions
+    {
+        public static string Name(this Period period)
+        {
+            string output;
+            switch (period)
+            {
+                case Period.LongBreak:
+                    output = "Long break";
+                    break;
+                case Period.ShortBreak:
+                    output = "Short break";
+                    break;
+                case Period.Work:
+                    output = "Work";
+                    break;
+                default:
+                    throw new ArgumentOutOfRangeException();
+            }
+
+            return output;
+        }
+    }
+
+    public enum Period
+    {
+        Work, ShortBreak, LongBreak
+    }
+}

PomoTime/PomoTime.csproj

@@ -132,6 +132,7 @@
     <Compile Include="MainPage.xaml.cs">
       <DependentUpon>MainPage.xaml</DependentUpon>
     </Compile>
+    <Compile Include="Period.cs" />
     <Compile Include="PeriodToStringConverter.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="RunningState.cs" />

PomoTime/RunningState.cs

@@ -4,33 +4,6 @@ using System.Runtime.CompilerServices;
 
 namespace PomoTime
 {
-    public static class PeriodExtensions
-    {
-        public static string Name(this Period period)
-        {
-            string output;
-            switch (period)
-            {
-                case Period.LongBreak:
-                    output = "Long break";
-                    break;
-                case Period.ShortBreak:
-                    output = "Short break";
-                    break;
-                case Period.Work:
-                    output = "Work";
-                    break;
-                default:
-                    throw new ArgumentOutOfRangeException();
-            }
-
-            return output;
-        }
-    }
-    public enum Period
-    {
-        Work, ShortBreak, LongBreak
-    }
     public class RunningState : INotifyPropertyChanged
     {
         public event PropertyChangedEventHandler PropertyChanged;