add menu option to already existing key bindings

[szabgab]

Insert TAB (Indent) TAB
Remove TAB (Outdent) Shift-TAB
Duplicate Current Line Ctrl-D

[claudio]

Additional information:
There are certain key binding that scintilla gives us "free of charge" and I think it would be important to let the users find these features. So in short a request to document and show these shortcuts.

[azawawi]

So basically Preferences/Keybindings should also allow the changing the "Scintilla commands"