Simplify title bar (#442)

Fixes #407 (partially) and #441

Description of the changes:
Remove TitleBarHelper and all <Border x:Name="CustomTitleBar" />
Let the system defines the draggable region
Centralize all events and functions associated to the title bar in a single control TitleBar instead of code splitted between MainPage/TitleBar/HistoryList/Memory.
Use the standard title bar when high contrast is activated instead of the custom one.
Modify the color of the title when the window doesn't have focus
Fix the right padding of the title bar with high contrast

How changes were validated:
Manually tested with LtR and RtL languages
Manually tested with high contrast
Tested when History and Memory flyout are opened

src/Calculator/Views/MainPage.xaml

@@ -13,7 +13,6 @@
       x:Name="PageRoot"
       Background="{ThemeResource AppChromeAcrylicHostBackdropMediumLowBrush}"
       Loaded="OnPageLoaded"
-      Unloaded="OnPageUnLoaded"
       mc:Ignorable="d">
 
     <Page.Resources>
@@ -38,11 +37,6 @@
     <Grid>
         <Grid.RowDefinitions>
             <RowDefinition Height="Auto"/>
-            <!--
-                This row is padding for the SystemFocusVisuals,
-                otherwise the focus rectangles render under the title bar controls.
-            -->
-            <RowDefinition x:Name="FocusVisualMargin" Height="3"/>
             <RowDefinition Height="*"/>
         </Grid.RowDefinitions>
 
@@ -89,14 +83,11 @@
                     Command="{x:Bind Model.PasteCommand}"/>
         </StackPanel>
 
-        <!-- Title Bar -->
-        <local:TitleBar x:Name="CustomTitleBar"
-                        Grid.Row="0"
-                        Visibility="Collapsed"/>
+        <local:TitleBar x:Name="CustomTitleBar" Grid.Row="0"/>
 
         <muxc:NavigationView x:Name="NavView"
                              x:Uid="NavView"
-                             Grid.Row="2"
+                             Grid.Row="1"
                              CompactModeThresholdWidth="Infinity"
                              DataContext="{x:Bind Model}"
                              ExpandedModeThresholdWidth="Infinity"