Add support for `shift left / right`.

[eddyxu]

Sometimes we copy and paste code from other sources, which has inappropriate indents. It would be great to support shift space operations in the editor, so we can add / remove indents for a large block of code.

[benweet]

Hi,
Have you tried to select your code and to use tab/shift+tab to indent/outdent?

[eddyxu]

Yes, it works! Thanks for your hints.

However, it might be helpful to have a toolbar item or a help entry within the "Shortcut" menu.