Add metadata block to replays

The replay files are just streams all network communication so to
get any info out of them it is necessary to play back the stream
until the wanted information is reached.

This introduces a new metadata block placed at the end of the
replay files and logic to read the new block, or fall back to
playing back the stream for older files.

The replay browser is also updated to use the metadata information
instead of reading the replay stream directly.

OpenRA.Game/OpenRA.Game.csproj

@@ -330,6 +330,7 @@
     <Compile Include="Graphics\PlayerColorRemap.cs" />
     <Compile Include="Graphics\Palette.cs" />
     <Compile Include="FileSystem\GlobalFileSystem.cs" />
+    <Compile Include="FileFormats\ReplayMetadata.cs" />
   </ItemGroup>
   <ItemGroup>
     <BootstrapperPackage Include="Microsoft.Net.Client.3.5">