Relax the selection requirement for indent/unindent selection

Before, the selection was required to span multiple lines. Now, any
selection will trigger the line indenting functionality of
Tab/Shift+Tab. This should lead to less surprises.

Also, Shift+Tab will now always unindent the current line, even if there
is no selection, since there is nothing else sensible for this shortcut
to do.

Task-number: QTCREATORBUG-414
Reviewed-by: mae