LineNumberMargin: align baselines of line number and main text

FoldingMargin: put marker in the center between TextTop and TextBottom, not in the middle of the whole TextLine.
IconBarMargin: put icon in the center between TextTop and TextBottom

src/AddIns/DisplayBindings/AvalonEdit.AddIn/Src/IconBarMargin.cs

@@ -100,7 +100,8 @@ namespace ICSharpCode.AvalonEdit.AddIn
 					int lineNumber = line.FirstDocumentLine.LineNumber;
 					IBookmark bm;
 					if (bookmarkDict.TryGetValue(lineNumber, out bm)) {
-						Rect rect = new Rect(0, PixelSnapHelpers.Round(line.VisualTop - textView.VerticalOffset, pixelSize.Height), 16, 16);
+						double lineMiddle = line.GetTextLineVisualYPosition(line.TextLines[0], VisualYPosition.TextMiddle) - textView.VerticalOffset;
+						Rect rect = new Rect(0, PixelSnapHelpers.Round(lineMiddle - 8, pixelSize.Height), 16, 16);
 						if (dragDropBookmark == bm && dragStarted)
 							drawingContext.PushOpacity(0.5);
 						drawingContext.DrawImage((bm.Image ?? BookmarkBase.DefaultBookmarkImage).ImageSource, rect);

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Editing/LineNumberMargin.cs

@@ -62,8 +62,8 @@ namespace ICSharpCode.AvalonEdit.Editing
 						lineNumber.ToString(CultureInfo.CurrentCulture),
 						typeface, emSize, foreground
 					);
-					drawingContext.DrawText(text, new Point(renderSize.Width - text.Width,
-					                                        line.VisualTop - textView.VerticalOffset));
+					double y = line.GetTextLineVisualYPosition(line.TextLines[0], VisualYPosition.TextTop);
+					drawingContext.DrawText(text, new Point(renderSize.Width - text.Width, y - textView.VerticalOffset));
 				}
 			}
 		}

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Folding/FoldingMargin.cs

@@ -116,8 +116,8 @@ namespace ICSharpCode.AvalonEdit.Folding
 			foreach (FoldingMarginMarker m in markers) {
 				int visualColumn = m.VisualLine.GetVisualColumn(m.FoldingSection.StartOffset - m.VisualLine.FirstDocumentLine.Offset);
 				TextLine textLine = m.VisualLine.GetTextLine(visualColumn);
-				double yPos = m.VisualLine.GetTextLineVisualYPosition(textLine, VisualYPosition.LineTop) - TextView.VerticalOffset;
-				yPos += (textLine.Height - m.DesiredSize.Height) / 2;
+				double yPos = m.VisualLine.GetTextLineVisualYPosition(textLine, VisualYPosition.TextMiddle) - TextView.VerticalOffset;
+				yPos -= m.DesiredSize.Height / 2;
 				double xPos = (finalSize.Width - m.DesiredSize.Width) / 2;
 				m.Arrange(new Rect(PixelSnapHelpers.Round(new Point(xPos, yPos), pixelSize), m.DesiredSize));
 			}
@@ -310,7 +310,7 @@ namespace ICSharpCode.AvalonEdit.Folding
 		
 		double GetVisualPos(VisualLine vl, TextLine tl, double pixelHeight)
 		{
-			double pos = vl.GetTextLineVisualYPosition(tl, VisualYPosition.LineTop) + tl.Height / 2 - TextView.VerticalOffset;
+			double pos = vl.GetTextLineVisualYPosition(tl, VisualYPosition.TextMiddle) - TextView.VerticalOffset;
 			return PixelSnapHelpers.PixelAlign(pos, pixelHeight);
 		}
 		

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

@@ -272,6 +272,8 @@ namespace ICSharpCode.AvalonEdit.Rendering
 							return pos + tl.Baseline - textView.DefaultBaseline;
 						case VisualYPosition.TextBottom:
 							return pos + tl.Baseline - textView.DefaultBaseline + textView.DefaultLineHeight;
+						case VisualYPosition.TextMiddle:
+							return pos + tl.Baseline - textView.DefaultBaseline + textView.DefaultLineHeight / 2;
 						case VisualYPosition.Baseline:
 							return pos + tl.Baseline;
 						default:

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

@@ -35,6 +35,10 @@ namespace ICSharpCode.AvalonEdit.Rendering
 		/// </summary>
 		TextBottom,
 		/// <summary>
+		/// The middle between TextTop and TextBottom.
+		/// </summary>
+		TextMiddle,
+		/// <summary>
 		/// Returns the baseline of the text.
 		/// </summary>
 		Baseline