hamburger button : visual update.

Change-Id: Ib4e305854443bf31fcf59f0f79d8f8120a8cc7ae
Tuleap: #790

MainPage.xaml

@@ -114,11 +114,13 @@
                                 <Button x:Name="_toggleSmartBoxButton_"
                                         Grid.Row="0"
                                         FontWeight="Bold"
-                                        FontSize="20"
+                                        FontSize="30"
                                         Content="&#xE700;"
                                         Click="_toggleSmartBoxButton__Click"
+                                        FontFamily="Segoe MDL2 Assets"
                                         HorizontalAlignment="Center"
-                                        Style="{StaticResource HamburgerButtonStyle}"/>
+                                        Style="{StaticResource TextBlockButtonStyle}"
+                                        Foreground="White"/>
                             </Grid>
                             <Frame x:Name="_smartPanel_"
                                    Grid.Row="1">