Doc: add missing info about sidebar views in Qt Quick Designer

As it turns out that we have a sidebar also in the Design mode,
with partly the same views as in the Edit and Debug modes, I
added info about the missing views and fixed the terminology.

Change-Id: I997d472548781a466bec3e64dbf28ecb1476b3eb
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>