Add developer shortcut to reload chrome files

Hitting Ctrl+Shift+C will re-initialize the chrome provider,
reloading all chrome files instantly. Useful when changing the
UI.

Note: A new "RootWidget" was created to trap top-level (global)
shortcuts instead of putting everything in Widget.

OpenRA.Game/OpenRA.Game.csproj

@@ -250,6 +250,7 @@
     <Compile Include="GameRules\RulesetCache.cs" />
     <Compile Include="Support\MersenneTwister.cs" />
     <Compile Include="GameInformation.cs" />
+    <Compile Include="Widgets\RootWidget.cs" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="FileSystem\D2kSoundResources.cs" />