File Extensions dialog - support .cue, also fix specific platform setting it looks like it never worked in the first place

BizHawk.Client.Common/RomLoader.cs

@@ -479,7 +479,15 @@ namespace BizHawk.Client.Common
 								case DiscType.TurboCD:
 								case DiscType.UnknownCDFS:
 								case DiscType.UnknownFormat:
-									game.System = "PCECD";
+									if (PreferredPlatformIsDefined(ext))
+									{
+										game.System = Global.Config.PreferredPlatformsForExtensions[ext];
+									}
+									else
+									{
+										game.System = "PCECD";
+									}
+
 									break;
 							}
 						}

BizHawk.Client.Common/config/Config.cs

@@ -46,6 +46,7 @@ namespace BizHawk.Client.Common
 			{ ".rom", "" },
 			{ ".iso", "" },
 			{ ".img", "" },
+			{ ".cue", "" }
 		};
 
 		// Path Settings ************************************/

BizHawk.Client.EmuHawk/config/FileExtensionPreferencesPicker.cs

@@ -28,7 +28,7 @@ namespace BizHawk.Client.EmuHawk
 				if (PlatformDropdown.SelectedIndex > 0)
 				{
 					return _availableSystems
-						.First(x => x.SystemId == PlatformDropdown.SelectedItem.ToString()).FullName;
+						.First(x => x.FullName == PlatformDropdown.SelectedItem.ToString()).SystemId;
 				}
 
 				return "";