Add tests for indent / unindent in normal mode.
Allow to add/remove multiple indents in visual

In normal mode, a count influences the motion following the shift
operator but in visual modes, the count signals the number of tabstops
to indent or unindent.

Reimplement InsertShiftWidth as a TextOperation to get acces to the
count.

Fixes #542.
Add tests for indent / unindent in visual mode.

Used to test fix for #542.