Update EditingCommandHandler.cs

OnTab and OnShiftTab doesn't handle Rectangle selection correctly (multicarret):
It is a customary behavior to insert/delete identation segments right before multicarret at each line, while currently AvalonEdit always inserts identation at the line's beginnings. Usuall selection (solid single/multi line)
behaves in old way.
My fix is a little bit awkward - i didn't dive into code to find out why rectangle selection becomes incorrect, I simply recreate it in OnTab