Support optional header in STFS, fix endianness in Descriptor (#80)

* Fix endianness in STFS Descriptor

* Support optional header for installer packages

* Fix field types

* Fix syntax

* Fix build

SabreTools.Data.Models/STFS/InstallerHeader.cs

@@ -0,0 +1,16 @@
+namespace SabreTools.Data.Models.STFS
+{
+    /// <summary>
+    /// STFS Optional header present in STFS packages for installers
+    /// Original research, field not mentioned on free60 wiki
+    /// </summary>
+    public class InstallerHeader
+    {
+        /// <summary>
+        /// String indicating type of installer
+        /// See Constants.InstallerType*
+        /// </summary>
+        /// <remarks>4 bytes, ASCII</remarks>
+        public byte[] InstallerType { get; set; } = new byte[4];
+    }
+}