Use Minecraft-Preview- prefix for preview builds

This change is made so that in the case that Microsoft publishes a
preview build with the same version as a beta build, the launcher is
able to have both of these downloaded at once.

Author: MCMrARM

MCLauncher/MainWindow.xaml.cs

@@ -531,7 +531,7 @@ public Version(string uuid, string name, VersionType versionType, bool isNew, IC
                 this.DownloadCommand = commands.DownloadCommand;
                 this.LaunchCommand = commands.LaunchCommand;
                 this.RemoveCommand = commands.RemoveCommand;
-                this.GameDirectory = "Minecraft-" + Name;
+                this.GameDirectory = (versionType == VersionType.Preview ? "Minecraft-Preview-" : "Minecraft-") + Name;
             }
             public Version(string name, string directory, ICommonVersionCommands commands) {
                 this.UUID = UNKNOWN_UUID;