Unstatic GlobalFileSystem and rename it to FileSystem

Add a ModFiles field on ModData and move all access to the file system to go through that.

OpenRA.Game/FileSystem/D2kSoundResources.cs

@@ -29,7 +29,7 @@ namespace OpenRA.FileSystem
 			this.filename = filename;
 			this.priority = priority;
 
-			s = GlobalFileSystem.Open(filename);
+			s = Game.ModData.ModFiles.Open(filename);
 			try
 			{
 				filenames = new List<string>();