Use strongly typed VsResourceKeys for styles

Change from using TextBlockEnvironment1111PercentFontSizeStyleKey to TextBlockEnvironment122PercentFontSizeStyleKey (which exists in VS 2015).

Author: jcansdale

src/GitHub.VisualStudio.UI/Views/Dialog/DialogStyles.xaml

@@ -1,12 +1,13 @@
 <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    xmlns:shell="clr-namespace:Microsoft.VisualStudio.Shell;assembly=Microsoft.VisualStudio.Shell.14.0"
                     xmlns:ghfvs="https://github.com/github/VisualStudio">
 
     <ResourceDictionary.MergedDictionaries>
         <ghfvs:SharedDictionaryManager Source="pack://application:,,,/GitHub.UI.Reactive;component/SharedDictionary.xaml" />
         <ghfvs:SharedDictionaryManager Source="pack://application:,,,/GitHub.VisualStudio.UI;component/SharedDictionary.xaml" />
         <StaticResource ResourceKey="ThemedDialogDefaultStylesKey"/>
     </ResourceDictionary.MergedDictionaries>
-    <Style TargetType="{x:Type TextBlock}" x:Key="GitHubH1TextBlock" BasedOn="{StaticResource TextBlockEnvironment200PercentFontSizeStyleKey}" />
-    <Style TargetType="{x:Type TextBlock}" x:Key="GitHubDescriptionTextBlock" BasedOn="{StaticResource TextBlockEnvironment111PercentFontSizeStyleKey}" />
+    <Style TargetType="{x:Type TextBlock}" x:Key="GitHubH1TextBlock" BasedOn="{StaticResource {x:Static shell:VsResourceKeys.TextBlockEnvironment200PercentFontSizeStyleKey}}" />
+    <Style TargetType="{x:Type TextBlock}" x:Key="GitHubDescriptionTextBlock" BasedOn="{StaticResource {x:Static shell:VsResourceKeys.TextBlockEnvironment122PercentFontSizeStyleKey}}" />
 </ResourceDictionary>