Fixed AvalonEdit bug: text cannot be dropped over a line (only in the small space between lines)

This was caused by the optimization that moved the text from the AE TextLayer to separate DrawingVisual controls:
Hit testing was disabled for the TextLayer, but we forgot to disable hit-testing for the DrawingVisuals

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Rendering/VisualLine.cs

@@ -454,5 +454,15 @@ namespace ICSharpCode.AvalonEdit.Rendering
 			this.Height = pos;
 			drawingContext.Close();
 		}
+		
+		protected override GeometryHitTestResult HitTestCore(GeometryHitTestParameters hitTestParameters)
+		{
+			return null;
+		}
+		
+		protected override HitTestResult HitTestCore(PointHitTestParameters hitTestParameters)
+		{
+			return null;
+		}
 	}
 }