fix SD-1394 - Indentation in region tooltip:

proposed algorithm:
Each line is checked for leading indentation whitespaces. If
a line has the same or more indentation than the first line,
it is reduced. If a line has is less indented than the first line
the indentation is removed completely.

See the following example:
	line 1
		line 2
			line 3
 line 4

is reduced to:
line 1
	line 2
		line 3
line 4

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

@@ -248,7 +248,7 @@ namespace ICSharpCode.AvalonEdit.AddIn
 					FoldingSection collapsedSection = foldingsAtOffset.FirstOrDefault(section => section.IsFolded);
 					
 					if (collapsedSection != null) {
-						args.SetToolTip(collapsedSection.TextContent);
+						args.SetToolTip(collapsedSection.TooltipText);
 					}
 				}
 				

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

@@ -2,6 +2,7 @@
 // This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
 
 using System;
+using System.Text;
 using System.Windows.Threading;
 using ICSharpCode.AvalonEdit.Document;
 using ICSharpCode.AvalonEdit.Rendering;
@@ -80,6 +81,63 @@ namespace ICSharpCode.AvalonEdit.Folding
 			}
 		}
 		
+		/// <summary>
+		/// Gets the content of the collapsed lines as tooltip text.
+		/// </summary>
+		public string TooltipText {
+			get {
+				// This fixes SD-1394:
+				// Each line is checked for leading indentation whitespaces. If
+				// a line has the same or more indentation than the first line,
+				// it is reduced. If a line is less indented than the first line
+				// the indentation is removed completely.
+				//
+				// See the following example:
+				// 	line 1
+				// 		line 2
+				// 			line 3
+				//  line 4
+				//
+				// is reduced to:
+				// line 1
+				// 	line 2
+				// 		line 3
+				// line 4
+				
+				var startLine = manager.document.GetLineByOffset(StartOffset);
+				var endLine = manager.document.GetLineByOffset(EndOffset);
+				var builder = new StringBuilder();
+				
+				var current = startLine;
+				ISegment startIndent = TextUtilities.GetLeadingWhitespace(manager.document, startLine);
+				
+				while (current != endLine.NextLine) {
+					ISegment currentIndent = TextUtilities.GetLeadingWhitespace(manager.document, current);
+					
+					if (current == startLine && current == endLine)
+						builder.Append(manager.document.GetText(StartOffset, EndOffset - StartOffset));
+					else if (current == startLine) {
+						if (current.EndOffset - StartOffset > 0)
+							builder.AppendLine(manager.document.GetText(StartOffset, current.EndOffset - StartOffset).TrimStart());
+					} else if (current == endLine) {
+						if (startIndent.Length <= currentIndent.Length)
+							builder.Append(manager.document.GetText(current.Offset + startIndent.Length, EndOffset - current.Offset - startIndent.Length));
+						else
+							builder.Append(manager.document.GetText(current.Offset + currentIndent.Length, EndOffset - current.Offset - currentIndent.Length));
+					} else {
+						if (startIndent.Length <= currentIndent.Length)
+							builder.AppendLine(manager.document.GetText(current.Offset + startIndent.Length, current.Length - startIndent.Length));
+						else
+							builder.AppendLine(manager.document.GetText(current.Offset + currentIndent.Length, current.Length - currentIndent.Length));
+					}
+					
+					current = current.NextLine;
+				}
+				
+				return builder.ToString();
+			}
+		}
+		
 		/// <summary>
 		/// Gets/Sets an additional object associated with this folding section.
 		/// </summary>