vendor: import KillerPDF 1.7.5 source (GPL-3.0) as the base for MMD PDF

Features/IShellServices.cs

@@ -0,0 +1,23 @@
+using System.Windows;
+
+namespace KillerPDF.Features
+{
+    /// <summary>
+    /// The three things every feature needs from the window: an owner for modal dialogs, string
+    /// lookup, and the status line. Each feature's own interface extends this, so the shell
+    /// implements these once rather than once per feature.
+    ///
+    /// Ported from Killendar, which is the reference implementation for this pattern.
+    /// </summary>
+    internal interface IShellServices
+    {
+        /// <summary>Owner for modal dialogs.</summary>
+        Window Window { get; }
+
+        /// <summary>Localized string for a Str_ key.</summary>
+        string Loc(string key);
+
+        /// <summary>Writes the status line.</summary>
+        void SetStatus(string text);
+    }
+}