Add a simple demo for using Gdk.Seat to query the mouse position.

Source/Samples/Sections/Miscellaneous/SeatSection.cs

@@ -0,0 +1,32 @@
+using System;
+using Gtk;
+
+namespace Samples
+{
+    [Section(ContentType=typeof(SeatDemo), Category = Category.Miscellaneous)]
+    class SeatSection : ListSection
+    {
+        public SeatSection()
+        {
+            AddItem("Press button to output mouse location:", new SeatDemo("Press me"));
+        }
+    }
+
+    class SeatDemo : Button
+    {
+        public SeatDemo(string text) : base(text)
+        {
+        }
+
+        protected override void OnPressed()
+        {
+            base.OnPressed();
+
+            var seat = Display.DefaultSeat;
+            ApplicationOutput.WriteLine($"Default seat: {seat}");
+
+            seat.Pointer.GetPosition(null, out int x, out int y);
+            ApplicationOutput.WriteLine($"Position: ({x}, {y})");
+        }
+    }
+}