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/Pak.cs

@@ -34,7 +34,7 @@ namespace OpenRA.FileSystem
 			this.priority = priority;
 			index = new Dictionary<string, Entry>();
 
-			stream = GlobalFileSystem.Open(filename);
+			stream = Game.ModData.ModFiles.Open(filename);
 			try
 			{
 				index = new Dictionary<string, Entry>();