add legend to dashboard.

This is a basic colour coding legend which describes the various states the dashboard can display.

src/EyesAndEars.UWP/EyesAndEars.UWP/MainPage.xaml

@@ -85,7 +85,27 @@
                                VerticalAlignment="Bottom"/>
                 </StackPanel>
             </Grid>
-            
+
+            <Grid Grid.Row="0" Grid.Column="2">
+                <StackPanel Orientation="Vertical">
+                    <StackPanel Orientation="Horizontal">
+                        <Rectangle Fill="SeaGreen" Width="92" Height="52"/>
+                        <TextBlock Text="Device is on" FontSize="42"
+                                   VerticalAlignment="Center" Margin="12,0"/>
+                    </StackPanel>
+                    <StackPanel Orientation="Horizontal">
+                        <Rectangle Fill="DarkRed" Width="92" Height="52"/>
+                        <TextBlock Text="Device is off" FontSize="42"
+                                   VerticalAlignment="Center" Margin="12,0"/>
+                    </StackPanel>
+                    <StackPanel Orientation="Horizontal">
+                        <Rectangle Fill="DarkOrange" Width="92" Height="52"/>
+                        <TextBlock Text="Unable to retrieve data" FontSize="42"
+                                   VerticalAlignment="Center" Margin="12,0"/>
+                    </StackPanel>
+                </StackPanel>
+                
+            </Grid>
             <!-- This device is not in use. Uncomment if becomes available. -->
             <!--<Grid x:Name="Factory3Grid" Grid.Row="0" Grid.Column="2"
                   Background="{x:Bind _vm.Device3.StatusColour, Mode=OneWay,