Add navigation and highlighting of change-blocks in Text Diff (#616, #717) (#703)

* Corrected misspelled local variable nextHigh(t)light

* Implemented change-block navigation

* Modified behavior of the Prev/Next Change buttons in DiffView toolbar.
* Well-defined change-blocks are pre-calculated and can be navigated between.
* Current change-block is highlighted in the Diff panel(s).
* Prev/next at start/end of range (re-)scrolls to first/last change-block
(I.e when unset, or already at first/last change-block, or at the only one.)
* Current change-block is unset in RefreshContent().

* Added safeguards for edge cases

* Added indicator of current/total change-blocks in DiffView toolbar

* Added new Icon and String (en-US) for Highlighted Diff Navigation

* Added Preference and ToggleButton for diff navigation style

src/Commands/Diff.cs

@@ -51,6 +51,9 @@ namespace SourceGit.Commands
                     _result.TextDiff.MaxLineNumber = Math.Max(_newLine, _oldLine);
             }
 
+            if (_result.TextDiff != null)
+                _result.TextDiff.ProcessChangeBlocks();
+
             return _result;
         }