Make IFolder interface inherently IDisposable.

Fix up implementations to ensure they dispose any stream they acquire, and ensure the constructor will not leave a stream open if it fails. Dispose folders when unmounting them in GlobalFileSystem.

OpenRA.Game/FileSystem/ZipFile.cs

@@ -17,11 +17,11 @@ using SZipFile = ICSharpCode.SharpZipLib.Zip.ZipFile;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class ZipFile : IFolder, IDisposable
+	public sealed class ZipFile : IFolder
 	{
-		string filename;
+		readonly string filename;
+		readonly int priority;
 		SZipFile pkg;
-		int priority;
 
 		static ZipFile()
 		{