Add internal branding corner to Universal terminal... (#4017)

... to convey that it's not ready for production.

src/cascadia/TerminalApp/defaults-universal.json

@@ -25,7 +25,11 @@
             "icon": "ms-appx:///ProfileIcons/{550ce7b8-d500-50ad-8a1a-c400c3262db3}.png",
             "padding": "8, 8, 8, 8",
             "snapOnInput": true,
-            "useAcrylic": false
+            "useAcrylic": false,
+            "backgroundImage": "ms-appx:///internal-background.png",
+            "backgroundImageAlignment": "bottomRight",
+            "backgroundImageOpacity": 0.4,
+            "backgroundImageStretchMode": "none"
         }
     ],
     "schemes":

src/cascadia/WindowsTerminalUniversal/WindowsTerminalUniversal.vcxproj

@@ -125,7 +125,6 @@
          so it's not in the common resource items. -->
     <PRIResource Include="Resources\en-US\Resources.resw" />
   </ItemGroup>
-
   <PropertyGroup>
     <!-- This is picked up by CascadiaResources.build.items. -->
     <WindowsTerminalAssetSuffix>-Universal</WindowsTerminalAssetSuffix>
@@ -135,7 +134,6 @@
     <WindowsTerminalAssetSuffix>$(WindowsTerminalAssetSuffix)Dev</WindowsTerminalAssetSuffix>
   </PropertyGroup>
   <Import Project="$(MSBuildThisFileDirectory)..\CascadiaResources.build.items" />
-
   <ItemGroup>
     <ClCompile Include="main.cpp" />
     <ClCompile Include="pch.cpp">
@@ -151,6 +149,9 @@
       <Project>{ca5cad1a-44bd-4ac7-ac72-f16e576fdd12}</Project>
     </ProjectReference>
   </ItemGroup>
+  <ItemGroup>
+    <Image Include="internal-background.png" />
+  </ItemGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
     <Import Project="..\..\..\packages\Microsoft.Windows.CppWinRT.2.0.190730.2\build\native\Microsoft.Windows.CppWinRT.targets" Condition="Exists('..\..\..\packages\Microsoft.Windows.CppWinRT.2.0.190730.2\build\native\Microsoft.Windows.CppWinRT.targets')" />

src/cascadia/WindowsTerminalUniversal/WindowsTerminalUniversal.vcxproj.filters

@@ -8,7 +8,7 @@
     <ClInclude Include="pch.h" />
   </ItemGroup>
   <ItemGroup>
-    <AppxManifest Include="Package.appxmanifest" />
+    <AppxManifest Include="Package-Dev.appxmanifest" />
   </ItemGroup>
   <ItemGroup>
     <None Include="PropertySheet.props" />
@@ -27,4 +27,7 @@
       <Filter>Resources\en-US</Filter>
     </PRIResource>
   </ItemGroup>
+  <ItemGroup>
+    <Image Include="internal-background.png" />
+  </ItemGroup>
 </Project>