Ground work for Wino Calendar. (#475)

Wino Calendar abstractions.

Wino.Core.UWP/Styles/ContentPresenters.xaml

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+<ResourceDictionary
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:controls="using:Wino.Core.UWP.Controls">
+
+    <!--  Flagged  -->
+    <DataTemplate x:Key="FlaggedSymbolControlTemplate">
+        <Viewbox
+            Width="14"
+            HorizontalAlignment="Center"
+            VerticalAlignment="Top">
+            <controls:WinoFontIcon
+                Margin="2,0,0,0"
+                Foreground="{StaticResource FlaggedBrush}"
+                Icon="Flag" />
+        </Viewbox>
+    </DataTemplate>
+
+    <!--  Attachments  -->
+    <DataTemplate x:Key="AttachmentSymbolControlTemplate">
+        <Viewbox
+            x:Name="AttachmentSymbol"
+            Width="14"
+            HorizontalAlignment="Center"
+            VerticalAlignment="Bottom">
+            <controls:WinoFontIcon
+                Margin="2,0,0,0"
+                Foreground="{StaticResource AttachmentBrush}"
+                Icon="Attachment" />
+        </Viewbox>
+    </DataTemplate>
+</ResourceDictionary>