Improved cheat file saving in GTK GUI. Extension .cht is now added if filter has it and filename is missing it. Also open cheat filename will be kept in memory.

git-svn-id: https://pcsxr.svn.codeplex.com/svn/pcsxr@83798 e17a0e51-4ae3-4d35-97c3-1a29b211df97