change "on foot" to "ground"

EliteBGS/MainWindow.xaml

@@ -108,7 +108,7 @@
                                                 <Separator Margin="2,0,2,0" VerticalAlignment="Top"/>
                                                 <Button Content="High" x:Name="High" Click="High_Click"/>
                                                 <Separator Margin="2,0,2,0" VerticalAlignment="Top"/>
-                                                <Button Content="On Foot" x:Name="OnFoot" Click="OnFoot_Click"/>
+                                                <Button Content="Ground" x:Name="Ground" Click="Ground_Click"/>
                                                 <Separator Margin="2,0,2,0" VerticalAlignment="Top"/>
                                                 <Button Content="Ship" x:Name="Ship" Click="Ship_Click"/>
                                             </StackPanel>

EliteBGS/MainWindow.xaml.cs

@@ -285,7 +285,7 @@ public partial class MainWindow : Window {
             return;
         }
 
-        transaction.Grade = "Low";
+        transaction.Grade = CombatZone.DifficultyLow;
         RefreshView();
     }
 
@@ -295,7 +295,7 @@ public partial class MainWindow : Window {
             return;
         }
 
-        transaction.Grade = "Medium";
+        transaction.Grade = CombatZone.DifficultyMedium;
         RefreshView();
     }
 
@@ -305,17 +305,17 @@ public partial class MainWindow : Window {
             return;
         }
 
-        transaction.Grade = "High";
+        transaction.Grade = CombatZone.DifficultyHigh;
         RefreshView();
     }
 
-    private void OnFoot_Click(object sender, RoutedEventArgs e) {
+    private void Ground_Click(object sender, RoutedEventArgs e) {
         CombatZone transaction = GetTransaction<CombatZone>(sender);
         if (transaction == null) {
             return;
         }
 
-        transaction.Type = "On Foot";
+        transaction.Type = CombatZone.GroundCombatZone;
         RefreshView();
     }
 
@@ -325,7 +325,7 @@ public partial class MainWindow : Window {
             return;
         }
 
-        transaction.Type = "Ship";
+        transaction.Type = CombatZone.ShipCombatZone;
         RefreshView();
     }