message: Fix rightmost tab corner
  Before and after:
  ![image](https://user-images.githubusercontent.com/78622729/111928492-710b9000-8abc-11eb-9760-0ecd3eb038fb.png)
  The bottom right corner appeared without a radius because of the custom padding.
diff: <SolidColorBrush x:Name="ErrorTextBrush"
  Color="{ThemeResource SystemErrorTextColor}" />
  - <!-- Suppress all padding except left around the tabs. The TabView looks far better like this. -->
  - <Thickness x:Key="TabViewHeaderPadding">8,0,0,0</Thickness>
  + <!-- Suppress top padding -->
  + <Thickness x:Key="TabViewHeaderPadding">8,0,8,0</Thickness>
  <ResourceDictionary.ThemeDictionaries>
  <ResourceDictionary x:Key="Dark">
  